I and others have posted Community Promotion Ads for the Literature proposal on several sites which are similar enough in scope that a significant proportion of their userbases might be interested:
Science Fiction & Fantasy
English Language & Usage (thanks @Helmar)
Movies & TV
Role-Playing Games (thanks @KitZ.Fox)
Each of these ...
There are plenty of literature forums already out there on the web, but our proposal has an advantage over many of these, which is the voting system.
I think placing advertisements in other literature forums, if they allow it, would be helpful. And if we are really bold, then we could ask permission to place fliers in libraries and/or bookstores.
There are two thoughts coming to my mind:
There's no better way of attracting the users to Area51 proposal than Sharing.
The proposal does not look mature yet — even after the remaining five three already one 200+ rep-users eventually join us.
Sharing is quite simple. It ensures that interested people will come to join. It is also important to have ...
For those enthusiasts of the proposed new site who are in academia, I would suggest a bold idea of socializing the proposal with literaryish departments of your local academia institution.
Post the ad on the bulletin board
Announce it at a student and/or faculty book club if one exists
Ask permission to announce at department meeting
Unfortunately, that is currently the only size available. The size setting in HTML isn't the best way to resize an image. Perhaps resizing it in an image editor will give you better results. You'll have a lot more control over the size, quality, and perhaps a bit of cropping and pushing around bits to better fit your needs.