I quote the following:
Change the slide background
If you want to apply the change to selected slides only, select the slides in normal view. Otherwise, the change applies to all slides that follow the design template of the currently selected slide.
On the Format menu, click Background.
Under Background fill, click the arrow on the right of the box, and then do one of the following:
Select a color from the color scheme
Click one of the eight colors displayed.
Select a color outside the color scheme
Click More Colors.
Click the color you want on the Standard tab, or click the Custom tab to mix your own color, and then click OK.
Select a fill effect or picture
Click Fill Effects, and then do one of the following:
To use shading, click the Gradient tab, click a type under Colors, click a shading style, and then click OK.
To use a texture, click the Texture tab, click the texture you want or click Other Texture to select a file and insert it, then click OK.
To use a pattern, click the Pattern tab, select the pattern you want, select foreground and background colors, then click OK.
To use a picture, click the Picture tab, click Select Picture to find the picture file you want, click Insert, and then click OK.
Use the background fill from the slide master
Do one of the following:
If you want to apply the background to selected slides, click Apply.
If you want to apply the background to all slides, click Apply to All.
From the MS Powerpoint Help facility, installed by default with the application.
You are so smart.lolHow do you change a flat tire?
Well, it doesn't take much ability to copy and paste a section from a help file does it ?desktop background help can't change to a picture
this answer prabably doesnt help anymore but either right click the paste button on your mouse or click insert at the top of the monitor.My computer background colour has changed all because of MSN.