This is an interesting idea, but I ran into a couple issues while trying it out:
- the drag-and-drop file interface on web works on chrome, but not firefox; I'd recommend using a file input tag so you can click to open a system dialog in addition to natively supporting drag-and-drop
- the generated pngs seem to be slightly corrupted; I was able to open them in a browser, but another image viewer gave the error "libpng: IDAT: Extra compressed data; libpng: Not enough image data"