[Help XD;]

Hello! I've been lurking for a long time now and I have come to love the tutorials given here so kudos to you guys! x3 ♥
I have a question though. I've been wondering how to do this little white background trick thing.
I know it must be a simple technique but for some reason I keep forgetting how to do it. XD

Here's an example;



I don't want it animated of course, I just want to know how to get that white sorta border around them
Much appreciation! ♥