When I start up my computer, I always have the same picture. How can I change it to something else?

if you mean the desktop wallpaper that is under the icons then go to start>settings>control panel>display>desktop tab.

you get several choices and click on the one you want.

If you want to automatically change your wallpaper each time windows start, you can use wallpaper changer program

Here's the example : http://www.vistanita.com/wallpaper-shuffler/

it's free !