Okay, so my brother had this kid in his game that he didn't like. So I went into the images file and edited it so that he didn't look so bad. I thought I had done a good job and was really proud of myself, but now there's a big, white block around all of the boys' heads. I've attached a picture. Does anyone know of a way to undo this? (The little boy is the one I originally changed. He had tufts of hair sticking out at his neck, and my brother said he looked like Princess Leia. )

What editing program did you use? I use paint.net. You need a program that supports transparency

Reopen the file in your image editor and save it as a transparent png. I use Photoshop but I'm learning to use GIMP (thanks to BayouBlonde).

This FAQ answer might help you.