The fictional character of "Buffy the Vampire Slayer" may be subject to copyright protection. The real person Sarah Michelle Geller is not subject to copyright protection. The real person Sarah, however, may have trademark protection in her name and likeness. Photographs of the real Sarah may be protected by copyright. Whether using such photographs to create a texture copies those photographs depends on what was done.

There are two ways of protecting oneself legally, if the texture is to be sold. One is to consult an intellectual property lawyer. Two is to contact the appropriate agent/trademark holder/copyright holder.

A while back, someone had a similar question about distributing free models of Marvel characters. Out of curiousity, I e-mailed the legal liason at Marvel, and got an answer as to what they thought acceptable (generally, they don't care about fans drawing Marvel characters for the fans' personal amusement, not for profit, and recognizing Marvel's ultimate ownership of the character). A letter that condones distributing a texture likeness is a lot more sure than guessing.