sorry for the late reply, but after finally getting some time to follow-up on this, I tried everything you said, and eventually got it to work (thanks by the way). But there's a smaller problem now. I tried adding a 3D plane as billboard, but it displays a red and black checkerboard pattern, instead of the sprite i tagged it as.
Ok, I understand now, but now i notice there's this problem, where I'm only able to create subfolders in the project files, instead of being able to import any png's. When i select "import files" it turns anything that i add to the project files into a subfolder, which really doesn't make sense, and has to be a problem on my end, because when i try it in other projects, it seems to work.