Is a checksum infallible?

Thinking about the checksums that are used to confirm the correct transfer of files through networks… I don’t know much about this but what I know lead me to this doubt… let’s say I have a bitmap image (hypothetical case) of 100000×100000 pixels all white and I do a checksum. If I paint one pixel black certainly the file will only be minimally changed. How sure can we be that the checksum will be different? Thanks!