Hm, I don't use GIMP so this is an iffy reply, buuuuut, for the pillars:
1) I noticed in the picture that the edges of the pillar/holes look a bit feathered. This adds a bit to making them look like holes, because it looks like the ground slopes down before it drops out completely. If you can get them to look a bit more defined, like the other room features, I think that'd help.
2) You could just say in the campaign that there are holes covered by illusory pillars (but don't warn the players), so when characters lean against them they fall down? It'd be good for a few chuckles, at least.
Beyond that I can't help you, but this looks really good, so if it's only your first I'd say your second will be amazing.