Okay, I've had 0 (AND I STRESS THIS, 0!) experience on designing tattoos, however, I have experience in using them and installing them.
First of all, if you want to use one in the downloads section here at Kayssplace, first install it and copy the tattoo you want into c:\program files\Lionhead Studios Ltd\Black & White\profiles\yourname\ - where yourname is the name of the profile you created). Note you may have installed your game in a separate directory. Also be sure the tattoo is numbered between 1 to 16. If you no longer want it delete it.
^ If this is what you wanted, more can be found in the readme.
However, if you want to know about designing tattoos, I'm of no help, maybe there's some FAQs here on Kayssplace? Read the readme as it comes with some tips and things you need to know about creating tattoos.
Hope this helped.