If you don't know the pixel resolution of a photo, it's easy to find out. Right-click it and choose Properties from the shortcut menu. Next, click the Summary tab, then the Advanced button. The dialog box will show the height and width of the photo in pixels.
You don't need to switch to the Windows Classic theme in order to set the Desktop to a solid color. Right-click the Desktop and select Properties. Click the Desktop tab. Under background, select None. Now choose a color from the Color menu. Click the Apply button to see your color on the Desktop.
Switch Themes p. 18
I have to admit, switching between Windows XP and Classic Windows themes isn't the most exciting way to customize your desktop. However, you can add more themes that will give you a variety of new looks for Windows.
There are many sites you can visit to buy themes and even download free themes. PC World magazine has a good selection of free themes at http://www.pcworld.com/downloads/.
Microsoft will sell you new themes as part of its $30 Microsoft Plus Pack. To get to Microsoft's Web page, right click your desktop to open the Display Properties dialog box, click the Themes tab, and select More Themes online.