Let's just say it's common practice to "take inspiration" from previously existing sources. Hell, sometimes you even steal from yourself. That being said, the older I get (didn't say wiser, now) I feel it's important to be forth- and upright (uptight? out of sight?) about my work. Even my latest album cover was sourced from creative commons materials.
Anyway, lots of stuff there free for the pillaging. Enjoy, artists.