I actually have other building sprites but I haven't imported them yet. Also I can't add more detail to the rain because Game Maker can only draw so many particles. I'll lower the fog a bit.
This is what I was concerned about; degrading the game's systems just to fit into the narrow allowances of the engine.
One idea to consider: have the rain sprite include multiple rain drops so you can get more drops on screen using far fewer sprites. Also, add an extra layer or two of rain of different scales to give the precipitation a sense of depth.
I'd recommend eliminating the fog. All it does is make the action and detail on screen hard to see, and it wastes valuable (apparently finite) particles.