Okay, quick tutorial.
The main terms in the online ad world are CPC (cost per click) and the CTR (click-through rate). As an advertiser, you want to keep your CPC low (around two or three cents per click) and your CTR high. (Really successful ads can achieve a .5% or even a 1% CTR rate.) You can track all of this in PW both for your ads AND for specific sites that you're bidding on.
Really, though, if you're thinking about what your ads should look like, it's simple: go for a single still image (no flashing GIF or anything like that), and make that image be something visually spectacular - like your recent voting incentive. If said spectacular visual has an insanely hot girl in there, that doesn't hurt (especially for ads running on TWC). Face it: your comic is gorgeous. Pictures from the comic sell themselves. It's way harder for a less attractive comic to reel in readers. Your ads are going to be amazing performers, I can tell you that right now.