Choose the caption bubble to begin. If you want to upload a custom bubble or alternative image then submit the caption first, then edit the comment and attach a "Place img". Once the image is approved you can use it by editing the caption again.
While I would usually attempt a witty comment about how hard the iTG Master Pimp NGE is trying to work teh bunny. I have to point out the red head in the background trying to squeez out the turtle head.