Different md5sum between the same file compressed two times

[EduardoThums]

I don't know exactly how the image is compressed under the hood, but it makes sense if the same file, compressed two times using the same options, generates the same md5sum?

I notice that even using the same options, the md5sums are not the same. Is this a specific characteristic of this compress implementation or its something normal under all the compress processes available, since they probably change some underling bytes, etc.

[Donaldcwl]

For jpg files, the checksums are the same for multiple compressions. For png files, the compression algorithm involves randomness, so it is normal to have different checksums.