THM can't be decoded
THM is a proprietary/undocumented format that no image library or converter can decode, so it can't be read as a source file.
Trying to open a THM as HEIC? Here's the honest answer — and how to get a real image.
Turn your file into a share-worthy AI Effect — one tap, no prompts.
THM is a small JPEG thumbnail saved alongside camera photos and videos — really a JPEG with a different extension.
Rename or convert THM to .jpg to open it anywhere.
HEIC is the high-efficiency format iPhones and iPads use to store photos — about half the size of JPG, but not supported everywhere, so it often needs converting.
HEIC opens on Apple devices and Windows 11; older systems need it converted to JPG/PNG.
No. THM is a proprietary thumbnail-cache blob that no image library can decode, so there's nothing to convert. Export the original photos and convert those instead.
THM files aren't meant to be opened directly. Recover the original images from your device's photo library or an iTunes/Finder backup, then work with those.
Once you have the real photos (JPG or HEIC), they convert cleanly to PNG, JPG or TIFF — free, no sign-up, no watermark.