Sounds like he got FTP then made a picture then saved it as the same file-name as the picture on the other PC then switched pictures so that when it reboots windows will find that file and open it only its a different image inside.

As for crashing... heck it doesn't take much to do that. You can entask the wrong programs, open missing or corrupted files or files on corrupted disks, compile & run programs with bad code, fill a batch file with a whole page full of garbage then open it, or you can sometimes even DoS a PC to a point where it can crash. Sure it helps to have a FW but for those run of the mill crashes that comes from programs you bought then it also helps to look at what the programs' requirements are such as mem, ram, OS type and (ect)