Go to personalize again
Click on Desktop background at the bottom
If you have more than 1 picture ticked, it will default to changing every 30 minutes. Untick all the pictures except the one that you actually want (You can only see the checkboxes if you hover over or click picture. If there are lots of pictures, you can untick them all by clicking in a clear area, then just tick the one that you one is ticked then the "Change picture every" option is greyed out.