I just bought a new iMac computer two days ago. Today I wanted to change the wallpaper so I found a photo I wanted to use on the internet, right clicked it and selected 'use as wallpaper'. I then decided I wanted to use a different one so I went into system preferences and went to change it that way however, when I click on wallpaper/screensaver it comes up with the window to change it but then freezes. It just says "loading photos" and the rainbow wheel spins around. I have to "force exit" to get out of it but everything else works fine over it.
Please help? I'm new to mac (obviously) and have no idea how to change the settings or anything.
Thanks :)

That should not be happening. Since it is so new, take it to the Apple store you purchased it at, and tell them about your problem. They will either fix it for free, or replace the computer for free.
That has never happened on any Mac I have ever used, that's not to say there wont be bugs from time to time.
This is probably not a critical error, but Apple customer service is usually very good.