Sorry for the late reply! I was actually able to make images that import right into SkyTemple! They'll be attached here and should work just fine.
I forgot to mention something else about importing images that might've also given you trouble. The game has strict color restrictions on backgrounds where in every 8x8 tile, each can only use colors from a palette of 16 indexed colors. Backgrounds can have 16 of these palettes for 256 colors in total. Meeting these restrictions usually involves converting the image through SkyTemple or manually changing it from your art program. I haven't used paint.net, so I can't offer too much advice on things related to that, sorry
Something else about this specific map to be careful if you don't know already: you'll have to reapply the Animated Tiles to the map's chunks once you import these images. In this map's case, it just makes the flowers move ingame. You can do this by going to Chunks -> Layer 2 -> Edit Chunks, and reapplying the Animated Tiles to the flowers.