Save the image of the graffiti that you'd like to use onto your desktop. Use your digital camera to take a picture of a favored piece of graffiti artwork, or find great photos of graffiti online. Choose an image 1024-by-768 pixels or larger, otherwise it might get blurry when you set it as your desktop (see Resources).
Open the image in the photo-editing software of choice. Almost any photo editor will have the features you need, though found on different menus.
Select the crop tool found on the "Edit" or "Image" menu. Set the proportion of the crop to 4-by-3 if your monitor is standard size, or 16-by-9 for wide-screen. A crop box appears on the photo.
Size the crop box with the anchor points on the corners and sides, and move it until the crop box surrounds the part of the image you want to see on your desktop. Press "Enter" to finalize the crop.
Save the image.
Open up your computer's appearances settings. Find this in the "Control Panel" on a personal computer, or through the "System Preferences" options on the Apple menu on a Mac. Optionally, right click anywhere on a Windows desktop and click on "Properties," and then on the "Desktop" tab. Once in the menu, click the "Browse" button to find the saved image, "Select" the image title, hit "OK," and exit. The graffiti image should appear on the desktop background.