1ITHMB can't be decoded
ITHMB is Apple's private iPod/iPhone thumbnail-cache format. It isn't a standard image file — it's an internal database of many tiny preview images packed together with no public specification, so no image library or converter can decode it.
2There's no picture to convert
Because nothing can read the ITHMB container, there's no image data a converter can extract and re-save as JPG. Any tool that claims to do it would just fail.
3Get the original photos instead
The real photos an ITHMB 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.