Copying Photos
Transfer photos from your computer to your PS3's hard drive via USB drive, or using a media server such as Windows Media Player or PS3 Media Server. If using a thumb drive, create a folder called "PHOTO" in all caps and copy the pictures into the folder; this sets the images to display properly in the XMB menu. Once you've connected to your media, go to the "Photos" menu and select it. Find the picture you want, press the triangle button and copy the photo to your hard drive.
Downloading Photos
Access the browser by going to "Network" and "Internet Browser." When you're at a website with an image you want to download, press the triangle button to view your options. Go to "File" and "Save Image" to save your desired photo to your PS3.
Taking Photos
If you have the PlayStation Eye accessory, you can take pictures and save them directly to your PS3 using the free EyeCreate utility. EyeCreate is a video editor, but it also allows you to take pictures. Once you've downloaded EyeCreate from the PlayStation Store, start it up and navigate to the camera mode. Choose the picture you want, click the square button to view your options and select "Export This Clip."
Setting Your Wallpaper
Once your desired background image is copied to your PS3's hard drive, navigate to it in the Photos menu and open it by pressing the "X" button on your controller. Once the image is displayed on your screen, press the triangle button and choose "Set as Wallpaper." Use the joystick on your controller to choose what portion of the photo you want to use as the background. Press "X" again to save the image as your background.