What is the THUMB file format?
.thumb files are thumbnails containing raster image data, typically stored as JPEG internally. They are created with the jAlbum application. Both the .thumb format and the software were developed by Jalbum AB.
A .thumb file is generated on the basis of a full-size image and is used to depict it in the program's gallery as a preview. Utilizing thumbnails enables more comfortable browsing through a gallery which contains many images. .thumb files enable placing multiple images on a single page, which saves the user's time.
Internally, jAlbum .thumb files begin with the JPEG SOI marker (FF D8 FF) and carry a MIME type of image/jpeg. Because of this, a .thumb file can usually be opened by renaming a copy to .jpg in any standard image viewer.
The .thumb format is proprietary, so the structure of files and the storing method are strictly defined by Jalbum AB. Note that the .thumb extension is also used by unrelated applications such as Snagit and certain game engines - always check the file header to confirm the origin.
Security & safety
RISK: LOWA .thumb is a passive image (thumbnail) and contains no executable code, so opening one is safe. The only cautions are practical: it is a low-resolution derived copy, not your original photo (don't rely on it as a master), and the generic .thumb name is also used by other apps - so before trusting one, confirm it actually contains image data (JPEG FF D8 FF header) rather than something disguised.
Format details
in a nutshell- Snagit thumbnail - TechSmith Snagit and various apps create generic .thumb preview-cache files unrelated to jAlbum.
- Game/engine thumbnail cache - Some games and 3D/CAD tools store level or asset preview thumbnails with a .thumb extension.
Programs that open THUMB files
Windows3 apps
JAlbum
Freemium jAlbum creates and manages .thumb files automatically as part of building an album; you don't open them manually. 
IrfanView
Free Open via File > Open and pick 'All files'; IrfanView often displays the JPEG data even with the .thumb name. 
Any image viewer (rename to .jpg)
Free Copy the .thumb, rename it to .jpg, and open it in IrfanView/Photos/Preview - it's a JPEG thumbnail. 
macOS2 apps
Apple Preview (rename to .jpg)
Built-in Copy the file, rename .thumb to .jpg, double-click to view in Preview. 
Linux2 apps
GIMP / any viewer (rename to .jpg)
Open-source Rename a copy to .jpg and open in GIMP, gThumb or any image viewer. Technical details
deep spec| Internal data format | JPEG binary data (begins with JPEG SOI marker FF D8 FF) |
| MIME type | image/jpeg |
| Magic bytes (hex) | FF D8 FF (JPEG Start Of Image) |
| Developer | Jalbum AB |
| Generating application | jAlbum web photo album software (available since ~2002) |
| Purpose | Auto-generated preview thumbnails for photo gallery navigation within jAlbum projects |
| Image compression | JPEG lossy compression (inherited from underlying JPEG format) |
| Proprietary format | Yes - format structure and storage method defined exclusively by Jalbum AB |
| Extension ambiguity | The .thumb extension is reused by unrelated apps (e.g. Snagit, some game engines) with different internal formats; verify via file header |
| Viewability workaround | Rename a copy from .thumb to .jpg to open in any standard JPEG viewer or image editor |
| Platform support | Cross-platform - jAlbum runs on Windows, macOS, and Linux |
| Storage location | Stored inside jAlbum project folders, typically alongside album thumbnails and cache data |
| Color model | YCbCr (standard JPEG color encoding used for photographic thumbnails) |
| File size | Typically small (a few kilobytes to tens of kilobytes, scaled-down preview dimensions) |
| Related extensions | .jpg, .jpeg (source format); .thm (Canon/Nikon thumbnail); .thumbnails, .thumbdata4 (Android thumbnail caches) |
| Released | jAlbum web photo album software (since ~2002) |
| Specification | jalbum.net |
THUMB conversions
Community Q&A
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