So, I realized that the dark, murky backgrounds combined with the many other colors of my Drawtober drawings were substantially inflating the PNG file sizes of the pieces, and that's what was preventing me from uploading higher resolution images.

To work around this, I've re-uploaded them as highest quality JPEG images instead, and they were below the 10MB file limit. So if you want to see those pieces again in all the detail that they were drawn in, feel free to take a gander.

It's not really that big of a deal and generally you shouldn't see much more than some added crispness, but it's something I'll be keeping in mind from now on.