I helped Hitch a bit with this, privately. We ended up with something like the following:
Which is pretty flexible: you can change the number of items in each row DIV and adjust the percentage size of the contained cell DIVs to match. One nice effect is that fewer cells in a row line up properly in the column. It also resizes nicely so that it fits well on different screen sizes. The only arbitrary choice was the img height in pixels.
Hitch needed the H3 tags for import into Sigil -- you can (and probably should) replace them with P tags.
Thanks to charleski for the basic idea and the suggestion for
{clear:both}. Hopefully someone else can make use of it, too.