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.
I was just about to comment, "whats with all the purri pics", but your comment says it all :). Carmac does the same thing you do though, you're not alone!
Who can blame you tho? =o