So depressed~~ Hero, come help quickly!

Today I downloaded a modified UC, tried it out, and felt it was pretty good, but the startup image didn't look nice, so I wanted to change it. I deleted the original startup image inside and replaced it with the one I wanted, making sure to rename the file the same way and even changed the default skin. But when I installed the modified software and opened it, you know the Java image appears? When I opened it, it just got stuck there. I waited for more than 5 minutes, and it was still stuck. I'm frustrated. Why doesn't it work? Can anyone tell me exactly where I went wrong?