Listen. If you look at Threadless and see a cool t-shirt, and the first thought that runs through your head is "OMG, I can make this so much funnier!" I want you to just freeze. Don't right click that mouse. Don't save that image to your desktop. Don't open Microsoft Paint. Just stop.
I won't sugar coat this for you - you are an idiot. I am not even going to bring up the fact that these are copyrighted images. I'm just going to point out to you that these images were funny before you decided to add your "hilarious" text and post them to Tumblr. In fact, everything you're thinking right now about writing over the image is making it less funny. You are ruining the joke in your effort to steal other people's work and pretend that you are clever.
You aren't. Just stop.
Listen, if you need attention on the internet that bad, there are better ways. Post a video of yourself to YouTube of your cat playing with a laser pointer. Pick a random subject and create a Tumblr blog titled "F*ck Yeah That Thing!" Create a podcast using the stock voice recorder on your phone of you and your friends laughing for 60 minutes while high. Then it will be you. You did that. You created something. Good job!
At the very least, when you look through Threadless for your next victim, link to the t-shirt. If you liked that shirt enough to paste your text all over it, you should like it enough to give the site a few hits from the reference when your three Facebook buddies click on it or when it gets reblogged on Tumblr.