Hi ferdicka,
No problem, there are no stupid question...

You don't say which desktop you are using, is it KDE?
I understand that you wish to change the desktop background. To do that, you can right-click on any empty space on your desktop, and a popup menu should show up. The last item in the menu is: "Desktop settings" or something similar. You can also use the "cashew" in the corner to get access to the Desktop settings.
There, you can choose the type of background: Image, and choose the picture you want. You have also other types of backgrounds if you like: pattern, slide show, earth globe, ...
Have a look at:
http://userbase.kde.org/images.userbase ... per-45.gifand
http://userbase.kde.org/Plasma/HowTo/4.5