On the extreme end, simply decode the image and reencode it using an encoder that you have vetted to not include any metadata.

But I agree, presumably the image data part of the file is well and exhaustively defined. I would be very interested in counterexamples that have practical consequences.

Note that there will still be concerns about stenography and fingerprinting which would warrant such a disclaimer from the creator of a tool aimed at a nontechnical audience.