I tried to put some wallpaper up. How do I delete it and get things back to the way it was right before that?

I tried to put up some wallpaper from a newsletter I received. It was there yesterday. Today I sign on and see a black screen behind all my icons. And, Crawler is my new opening page. I wand to to go back to yahoo. I really want everything to go back to the way it was before I got so inquiring.