If I make a user upload a picture during the signup process on a website, then make them upload a similar photo during the login process, how can I check or validate between those two images https://1921681254.mx/?
What exactly do you mean? Compare them to check if they are the same or different?
If the files are the exact same size chances are they are the same (I’m talking bytes). They may not be, but it is quick to check.
You can create a hash of the files (like an MD5) to see if they are the same. But that has nothing to do with the actual image. Two images can contain the same subject and not have the same hash.
If you want to check that two images contain the same subject you would have to do some image processing. I’m sure there are libraries (pixelmatch).