From the image I can see that you don't have your Tool Box widget (unnamed) in the corner of your screen.
Maybe the transparent image (above and bellow) the K3b image has to do with this
To move the Tool Box widget, right-click > "Unlock Widgets", drag-drop it to the corner and right-click > "Lock Widgets".
True I don't have the toolbox widgit named but I didn't think this would have anything to do with it but.....then again I am new to kde so I will fiddle with that when I get home.
I just happen to have the k3b open at the time I took a screenshot for example....The lines appear anywhere on the empty desktop...not just in corners etc.