Try using PaintshopPro, the latest version is avaiable as a shareware download for free, or if you have access to PCAnswers this month they have a free copy of version 5. Also you could try Photoshop but this is not a cheap solution. It is quite easy in the basic form. start off with a new sheet sized to 1024x768, fill background with required colour and then paste in your pictures etc, not forgetting to save frequently.
Photoshop is the best tool for virtually anything concerning image editing, but it's very expensive. I have bought every version from 5.0 to 7.0 and am very pleased with it. Basically follow what thargor_ says, but you might wish to read some free tutorials on Photoshop. Just google for "photoshop tutorials" or something.
