I've submitted the accepted logo below to Math, IT Security, and CS Theory. Please go to those sites (use links to meta post below) to upvote so our ad will appear on those sites. If there are other sites we should consider, please submit them as an answer.
Also, don't feel like you can't submit a new logo if desired. If one gains more votes, I'll switch to that.
Graduated SE sites each have a meta question where users can submit ads, which if upvoted at least 6 times will be displayed on the main site. This provides a good way for us to promote Crypto.SE on other SE sites.
Which sites should we target for ads?
What ad should we use (i.e., we don't have one AFAIK, so design one, post it here and see what other people like)?
When we submit an ad it is of the form
[![Tagline to show on mouseover]] : http://image-url : http://clickthrough-url
So please post this information in your answers to make it easy to add on to the target sites.