I just found an edge-cutting method of creating 16-bit color backgrounds in JJ2+!
Essentially, you subdivide an image in its cyan, magenta and yellow values, then you put them in front of eachother, and on sprite mode you choose Blend Multiply.
With this little trick, I managed to insert a 16 bit color image in a level. Here's what it looks like:

The only downside is that it doesn't work in 8 bit mode, but who uses 8 bit mode nowadays?
To prove you the image isn't counterfeit, I linked the level and tileset.
https://www.dropbox.com/scl/fi/7fye0...=uddu5hnk&dl=0