Hi Andrew,
There is no official logo, mascot, trademark, or advertising campaign for TLA+. The official "spelling" of TLA+ is with the "+" a superscript. There is the splash page, which is in the file splash_small.png in org.lamport.tla.toolbox.product.standalone/images on the CodePlex site. It was drawn by my brother, so if used, I'd appreciate it if "(c) Edward Lamport" appeared with it.
Leslie