I also thought about that. But unless I know how they create the checksum, I cannot reproduce it.
The most simple would be XOR but there is also CRC etc.

I guess without further inside info we are screwed or can just try.
My program replaces the images very exactly. So if someone can exchange the image with a smaller one (filesize) and it works, then we know that there is no checksum.
With bigger file is always the risk to "disturb" the bytes of the next file.