Let me ask you this. I bet in researching your new tattoo you have already gone to Google images to find pictures. Don't you think other people have already done this? Do you want to end up with the same tattoo as someone else or do you want something unique? I know the reason that you are getting is tattoo is to stand out and be unique. So make sure you do your research before getting your new tattoo.
There are a lot of free galleries online dedicated to tribal tattoos. But I would highly recommend that you use a pay site because of the features that they offer. If a site is free everyone else is going to be using it. Paying for a site gives you access to great tattoo artists as well as a community of dedicated tattoo enthusiasts. You will also get access to an updated list of quality local tattoo artists. You want to find a tattoo artist that has experience and runs a clean shop. You are about to spend a lot of money and time, so do yourself a favor and do plenty of research before you get your new tribal skull tattoo.