Chris, it's VERY Important that you have something done that can be Reproduced as a sticker/decal. I tried to explain this to people on here before.
There is a big difference between Cool Artwork and Usable Artwork. Very detailed artwork is more appealing, but it doesen't always dransfer to a usable sticker/decal. If you go with a Screen Print style sticker/decal, you can get away with a lot more, but they will charge you setup fees for multiple colors. If you are going with a Cut Out sticker/decal, detail is Not your friend.
There is a ton of cool skull black line artwork on the internet. They make for a Great starting point that can be added too. Everything from simple to Highly detailed.