Hedi, you're not a dumbbell at all.
Why don't you try saving one of the avatars to your pc, and the uploading it to the site? It might be easier.
Let's see if I can explain how simply enough...
1 - Decide which cute avatar you want.
2 - Right-click on the avatar and go to "Save Image As"
(Just remember to notate what the name of it is, and where you saved it on your pc, such as in 'My Documents' or wherever.)
3 - Click on the "Options/Usercp"
button up on top of the site.
4 - Click on "Edit Options"
5 - Scroll all the way down to the bottom of that page and you'll see where the Avatar option is. Over on the right, there's a "Change Avatar"
button. Click on that.
6 - Scroll down on that page. At the bottom you'll see "You can upload an avatar from your computer:"
on the left. To the right of that, you'll see a "Browse"
7 - Click on that "Browse"
button and it'll show you your computer's file system. Go to where you saved your new avatar and choose it.
8 - You may have to hit "Okay"
after that. I didn't actually go through changing mine, so I can't be absolutely sure.
Hope that helps.