You can either install a wallpaper app from the Google Play store or use your browser to find images and download it to your tablet to use as a wallpaper.

If you want to download using your browser here's how to do it:
Using Google Chrome, find an image you want to download, then view it. Tap and hold the image and choose open image in new tab. Now tap and hold the image again and choose Save image. And there you have a new image to use as a wallpaper. :)