How to get rid of slide show on desktop?

I have an iMac G5. I don't how, but I inadvertently changed the desktop background picture when I was trying to change the screensaver. Now the desktop switches pictures every 5 seconds(rotates 3 of them) I tried selecting the desktop picture I wanted, but it only gets added to the slideshow with the other pictures I can't get rid of.Also these pictures don't appear in the window on the right of the desktop part of the desktop & screen saver window. The 'change picture' box isn't clicked on. The picture I want appears in the small window when I select it, but only stays on the screen 5 seconds before the others return. In 2 years this has never happened before. I don't know what to do. Help!