Ugh, I hate HTML emails. Anyway, I can't tell from what's going on with the fiddle, but I think I can guess as to what's happening.
Some email clients strip your DOCTYPE declaration so I guess that's why you didn't declare a doctype. But, if a page has a strict DOCTYPE, images will have this "gap" beneath them. One solution is to set the image as a block-level element:
I would suggest reading more about that here
. It talks about "images, tables, and mysterious gaps".