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 turn a THM into TIFF? Here's the honest answer — THM isn't a readable image format — and the steps that get you a real picture.
We can't convert THM to TIFF — an THM file can't be opened by any converter. Here's why, and what actually works.
THM is a proprietary/undocumented format that no image library or converter can decode, so it can't be read as a source file.
Because nothing can read the THM container, there's no image data a converter can extract and re-save as TIFF. Any tool that claims to do it would just fail.
The real photos an THM cache was built from live in your photo library or a backup. Export those (they'll be JPG or HEIC), then convert them — that's a conversion that works.
THM is a small JPEG thumbnail saved alongside camera photos and videos — really a JPEG with a different extension.
How to open Rename or convert THM to .jpg to open it anywhere.
Full THM format guideTIFF is a high-quality, often lossless raster format used in photography, scanning and print. Files are large but preserve maximum detail.
How to open TIFF opens in Photoshop, GIMP, Preview and professional imaging software.
Full TIFF format guide