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.
but then again i realized i've been smiling for some time browsing the gallery, it must be the effect of everyone happily grinning in every picture, i'm too gullible dammit!
especially Ms.X's smile is infectious ^^