Free tool
SHA-256 hash checker
A ThreadProof packet lists a SHA-256 fingerprint for every source file. Drop the original file here to recompute that value and see whether the copy you hold is identical. Files are read in this tab and are not uploaded.
Drop files here, or
What this does and does not tell you
A matching value means the two files are byte for byte the same, so the copy has not been re-saved, cropped or re-compressed since the packet was made. A different value means the files differ, though that can be innocent: some messaging apps re-compress an image when it is forwarded. Neither result says anything about what happened on the phone before the screenshot was taken.