Are you on Windows or Mac? And do you want a screensaver or a wallpaper?
If you're on Windows and want to use your image as a desktop wallpaper, then you can right click on the Desktop and click 'Properties'. The tab second from the left is 'Desktop'. You click on 'Browse' and select your photo. Under that is 'Position'. It can be either 'Center', 'Tile' or 'Stretch'. If you select 'Stretch', and it sounds like you have, you'll want to change it to 'Center' instead.
Then click on the 'Appearance' tab. Click on 'Advanced', and then click on the background colour in the window (default is dark blue). Where it says 'Color 1', click on that and select 'Other'. From there you set the HSL or RGB values, or simply mouse around to the colour you want, and add to custom colours. Select that one, and it will change the background colour (behind your image) to that colour.
Mike.