Is there something inherently wrong with being able to upload more than one image per post? Not necessarily with Wakaba, but with image boards in general. Perhaps something like this was tried somewhere in the past, and it didn't work out, due to being too much of a server load, or...? To me, it seems like a fairly straightforward thing to implement (if you know what I mean).
Ochiba lets you upload ZIP archives of images, but then it just creates a new post for each image.