I tried using XnView MP, PIL, and OpenCV to stitch together the tile map I needed. I found that for levels 7 and 8 (32768*65536, 65536*131072) output PNGs, there is a strange phenomenon in the preview in XnView:
At 100% zoom, the bottom 1/3 of the content cannot be displayed normally, and the further down you move, the greener it gets.
This only happens at 100% zoom, not at 150% or 50%.