I usually use caps from the shows (often Ashley); movie stills or memes that made me laugh. Right now it's Ed asking this question from Shaun Of The Dead...
(click for a larger image)
Alternating between these two at the moment...
...as the mood takes me
^^^^^^^ Oh my, I would not be able to concentrate.....
I've changed mine to this which I thought was very atmospheric.
'Fraid I stole the idea from Sm© from a few posts back