Yes. You can download and use them as you like as far as you don´t make money out of them: they´re free for personal, non commercial use.
However, if you choose one as inspiration for your tattoo, we ask you to modify the design according to your taste and personal story, customizing it as you like... this way every design, yours included, will remain unique and personal!
For commercial use, you should ask for explicit consent by their authors and owners, under the conditions established by them.