KDE-Look.org is the premier site for styles, themes and other goodies for your KDE desktop. One of the goodies offered there is wallpapers (or desktop backgrounds, if you prefer).
Obviously, they are not specific for KDE usage, as they are just standard image files, but it is nice to have them there. You can download them from the website, but for KDE users there is a better way...
Just as if you were normally changing your desktop background, right click the desktop and choose Configure Desktop. On the Background tab, you will notice a button called Get New Wallpapers.
If you click this, it will load the latest wallpapers straight off the KDE-Look website. You can browse through and preview them straight from this window. If there are any you'd like, simply click Install to download them.
They will now show up in the list of pictures labelled Picture: back in the main window. Just pick the one you just downloaded and you can apply it straight away to your desktop.
This little integration feature of KDE goes too easily unnoticed - and if you like a change of background every now and then, this can be a way to find something cool and apply it, all in just a few clicks.