Question is listed as one of the "The 7 Essential Meta Questions of Every Beta":
This one is pretty straightforward. Solicit contributions, throw out ideas, post preliminary (or finished) designs, and be supportive and respectful of other people’s ideas and creativity.
We have designers on staff who will actively help come up with site designs but, if an idea stemming from the community stands out as exceptional, we are happy to use it.
- Original artwork only, since the idea is for the suggestion to be usable as is.
- Have a copy of the logo in a vector format, but post it as a PNG or JPG below as a preview.
- Review how other betas approached finding a logo.
- Here's a post by Jin, designers on SE staff referenced above, on the approach he took on the logo, design, etc on an SE site.