In the general sense of how you would change any icon in Finder, it might work to select the project, press ⌘I to get info, and then copy an image you want to use for the icon, click the picture of an icon in the top left of the Info palette, and paste.
That will just change the icon though, if you have Finder set up to show content previews then I think there is no control over that.
.:. Ioa Petra'ka “Whole sight, or all the rest is desolation.” —John Fowles