When posting a photo, many people look only at what is visible in it.
Whether a face is visible. Whether an address is in the background. Whether a name tag or car license plate can be seen.
Of course, that is important.
However, images can retain information that is not visible on the surface. This includes capture time, camera or smartphone model, editing software, and in some cases GPS location information. This information may be stored as metadata attached to the image file.
If you are thinking about anonymity, check both the "appearance" and the "internal information" of a photo.
This article organizes what and GPS information are and why they affect anonymity.
The actual check procedure before publishing photos is covered in "EXIF and GPS Checks Before Publishing Photos."
What Is EXIF?
EXIF is capture information that may be included in photo files.
Images taken with smartphones or digital cameras may store capture time, camera model, lens information, orientation, exposure settings, editing software, and similar information. If location recording is enabled, GPS information may also be included.
Information
What can be known
Effect on anonymity
Capture time
When it was taken
Connects with posting time or activity history
GPS
Capture location
Reveals home, workplace, school, or the site
Device model
Smartphone or camera being used
Becomes material for correlation with other images
Editing software
Editing environment
The usage environment or work device is inferred
Image orientation and size
Capture or editing state
Becomes a clue to the same device or same work process
Not every image retains the same information.
Some apps and social networks remove metadata at posting time. On the other hand, cloud sharing, file transfer, bulletin boards, and email attachments may leave the original metadata intact.
What matters is not leaving it to the service.
GPS Information Is Especially Dangerous
When GPS information remains, the capture location may be directly visible.
A photo taken at home, a photo of materials at work, a whiteboard photographed at school, a photo taken at a meeting place. Even if the place is not visible from the appearance, GPS data can reveal where it was taken.
Image
What happens if GPS remains
Photo taken at home
A daily-life base is revealed
Photo of workplace materials
It connects to affiliation or workplace
Event photo
Participation place and time are known
Travel photo
Movement history and life patterns are visible
Reporting photo from the field
Sources or the scene are inferred
GPS is strong information even by itself.
When combined with posting time, body text, past posts, and other photos, it also reveals routine places and behavior patterns.
Internal Information May Remain Even When the Image Looks Safe
Even if you blur the background of a photo, the capture place or time may appear if EXIF remains.
Conversely, even if you delete EXIF, a sign in the background, the view outside a window, reflections, clothing, weather, shadows, notifications, or the subject of the photo may reveal the place or time.
Check target
Information to look at
Point to watch
Appearance
Faces, background, reflections, signs, name tags
Information visible to the eye
Metadata
GPS, capture time, device, editing software
Information not visible from appearance
Post text
Place, time, situation description
Combines with the image
Posting time
When it was published
Connects with capture time or local information
For anonymity, either one alone is not enough.
Check both the appearance and the internal information of images.
ExifTool Can Check It
ExifTool is a representative tool for checking image metadata.
ExifTool is a metadata checking and editing tool by Phil Harvey, and it can handle many metadata formats such as EXIF, GPS, XMP, and IPTC. It is used not only for images but also for checking information in a wide range of files such as video and audio.
The reason for introducing ExifTool is not simply that it is well known. It has been used for a long time, supports a wide range of formats, and can check detailed metadata from the command line.
However, using ExifTool does not make the photo safe by itself. Even if you can check metadata, it does not automatically protect the image appearance, post text, cloud sharing settings, or past information.
Order for Checking
Before publishing a photo, check in the following order.
Look at the appearance of the image
Check EXIF and GPS information
Create a publication copy
Open it in a different environment and see whether metadata remains
Check whether there is a problem when combined with the post text and posting time
Stage
What to check
Appearance
Faces, background, reflections, text, place clues
Internal information
GPS, capture time, device, editing software
Publication copy
Do not publish the original image as-is
Recheck
Open it in another app after removal and check
Correlation check
See whether it connects with the body text or posting time
Even when a social network or messaging app removes metadata, it is important to build the habit of checking the original image before upload.
Behavior can change when the posting destination changes.
Do Not Overtrust Service-Side Removal
Some social networks and messaging apps remove image metadata at posting time.
This is convenient, but leaving anonymity to a service is dangerous. The removal scope differs by service. Behavior may also differ between posting something as an image and sending it as a file attachment. With cloud sharing, the original file may be delivered to the other person as-is.
Sharing method
Point to watch
Social network post
Metadata may be removed, but behavior differs by service
Messaging app image send
It may be removed through compression or conversion
File attachment
Original file metadata may remain
Cloud sharing
Often shares the original image as-is
Email attachment
File names and metadata tend to remain
Rather than remembering "this app should remove it," it is more stable to create and check your own publication copy.
Recheck After Removal
Metadata removal is not finished just because you ran it.
Some information may remain even when you think you removed it. Editing apps may add new metadata. When an image is saved again, creation software or save time may be newly added.
After you create a publication copy, check it again.
What to check
Reason
Whether GPS is gone
Location information is often the most dangerous
Whether capture time remains
It connects with activity time
Whether device names or software names remain
It becomes a clue to the work environment
Whether the file name has a problem
Even if metadata is removed, the name can leak information
Whether the image appearance contains information
Check clues other than internal information
Treat checking, removal, and rechecking as one procedure.
Summary
EXIF and GPS information are metadata that may remain inside photos.
Capture time, capture location, device model, editing software, and similar information affect anonymity. GPS information in particular can directly indicate a home, workplace, school, site, or reporting location.
Before publishing a photo, check not only its appearance but also its metadata. Tools such as ExifTool help with metadata checks, but they do not automatically protect image content, post text, posting time, or cloud sharing settings.
For anonymity, it is important to check photos from both "visible information" and "invisible internal information."
Related tools
Reverse image search
Google Lens
An external resource related to this article. Open it only when it fits your situation and threat model.
Why it is listed: It can help with the article topic, but it is outside Anonymity Sense and should be checked before use.