Ok I'm marking Anna's response as the answer because though I didn't fully test it, it seems like it should work and answers the original question with jsx. However I'm also leaving my final solution in case anyone else runs across this with the same issue and may prefer this method as well.
What I ended up doing instead is using InDesign. I figured out that it has a grid option that lets you import a number of files and place them all in an equal grid in a single command. This is almost exactly what I was looking for, except that it leaves a small border/margin in between the columns and grids and mine were designed to meet exactly.
I couldn't figure out how to make it not have the border (I have very little experience with InDesign, it may be possible). However I was able to select all my images and scale them uniformly to be the correct size, then I just selected each column and dragged it over to snap to the adjacent column and the same with rows...