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.
Apparently the 'my head just got run over by a lawnmower' look is coming back into fashion. Add that to the already popular 'I have an onion ring dangling from my ear' look and you've got yourself a winner.