One of the first times I did it, was 15-20 years ago. I had a digital camera, a Canon IXUS S110, and wanted to have some pictures printed. There were some sites where you could upload pictures, but they all used som proprietary Windows program, and I had Linux. Except the supermarket chain Føtex which had a website with a Java applet. But it failed when I tried to use it throwing an exception about a missing directory.

So I downloaded the JAR file, decompiled it with the JAD decompiler and found that they had hardcoded the file separator as backslash. So that was easy to fix. I compiled it, ran the JAR file and uploaded my pictures and picked up the prints a few days later.