I got around to fixing the annoyances from the first release:
Added an 8x8 grid around each tile so it's easier to see palette boundaries
Added saving/loading of in-progress images
Increased max size of image preview frame to 2000x2000
Shoutout to Chris Ahchay for being the only person to download my initial release, his blog can be found at https://ahchayblog.wordpress.com/blog/ and he has some pretty good tutorials for getting started with programming on the NGPC if you're into that.
Also note that I'd recommend putting tiles into the console's tile memory starting at index 1, as the second background layer initializes to be displaying all tile 0 so if it's not blank your image will look strange.