Tap the Settings app and then select Brightness and Wallpaper. There, you'll find two options. First, you can enable auto-brightness, which adjusts the display's brightness by monitoring the environment's ambient light.
Tap the images below to bring up the wallpaper settings. At the top of the page, you'll find the iPad's default images followed by your own pictures (if you've synchronized photos). Tap any image to bring up the full-screen view. Now, you've got three options: set the lock screen, home screen, or both. You're done!
There are two things that you should note. The first thing is that your images are sorted by album, events, and then faces, which makes searching easier. Secondly, you can't adjust or re-size an image before setting it as a wallpaper like you can on an iPhone.
*That's going to change this summer.