Photoshop would be my guess. You could take screenshots of the objects in Unity, on maybe a flat background. Take it into photoshop, and remove the background. Add in any designed background you'd like -- a gradient, a drawn fire thing, whatever works. (I'm not a graphic artist myself, so I have no idea how to design those things). You could then add more layers to spice up the images even more. The end result should be icons that match the in-game models, but have whatever extra flair you'd like. Alternatively, post on the Non Commercial Collaboration forum and see if anyone who is into that type of stuff would want to work with you on it.