Go to pixlr.com (if you don’t have Photoshop because that shit is expensive). Find a background image and upload it, e.g. pizza.
Open another image and erase all the extra crap around it, e.g. me running!
Resize the new image a couple times. Paste each resized layer onto the background and save 4 or 5 images.
Upload images to picasion.com or any other gif making site (you can also download Gimp but to keep things simple lets just use the free sites). Download the gif, upload it to Tumblr. TA DA! You now have a gif of yourself jumping into pizza!