I prefer text, but I can tell you now that graphic pieces serve as a stronger imprint in someones mind after they leave the thread. Which is why they work well.
Moreover, they can create the perception of a "brand" if they are created well. This all ties into stronger purchasing signals. Therefore, it is in the interest of a BST creator to utilize visual stimuli to some extent. My previous BST was a very short graphic. It mostly only converted senior members.
Oh, and it is easier to pick up, excite and entice newbies with what I've stated above. Lots of colors, sharp, flat images, and bold promises.