Can I use 2.5D graphics? Not like Diablo 2's isometric, but more like or the one on the Aveyond game series or most RPG maker games.
I do understand what you are talking about. Most 2D games do actually use 2.5D graphics. They show the traditional height and width, and attempt to represent the third dimension depth. Almost all RPG Maker games have this 2.5D imaging system.
In my personal games, I like to eliminate this .5 dimension, and make what I call 'Tru 2D' images, using only height and width/length, with no depth representation at all. This also means no shadows. I have made one true top view 2D game where only length and width were represented, with no height.
Answering your question, yes 2.5 images are allowed since they attempt to represent the 3rd dimension but actually are not 3D images. If you could send me a sample image to email@example.com, I could easily verify if we are talking about the same image style.