We have tons of backgrounds on this site, both seamless tileable backgrounds and wallpapers. Browse through the galleries and check out the previews until you find one you like.
When you click on the image or press sample for desktops (1), you get a popup screen with a preview. Click on the coloured boxes a bit to see how the background works with different text colours.
Found the background you like? Click on "View of download this image" or a resolution (2).
Then right-click on the image and choose "save image as..." to save it on your own computer. It doesn't matter where you save it, just remember where you put it.
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It's best to upload the background to a free image hosting service so your profile
is always going to work.
Go to the website 
Now the fun begins! Go to the Edit profile screen and click the second
tab Customize. We need the part Advanced users where we use CSS to set
the background.