I think the every six weeks suggestion is good. We could eliminate all the overlap of choosing the theme for the next contest while the current one is still going. The timeline could be something like this:
week 1: accepting suggestions for a theme
week 2: vote on the theme
week 3-5: work on your photos and submit them
week 6: vote for our winner!
Now, the other problem is how we seem to have too many threads going about the contest, and it's difficult to find information. my suggestion is that the whole thing should be moved to the photography and graphics board for starters.
For each period, we can start a thread for suggestions, and then after one week, add a poll to that thread to vote on the theme, rather than opening a new thread with a poll. The theme will then be announced in that same thread, along with the deadline for submitting entries, and the email address to send them to. This topic can be set as a sticky topic from the very beginning, and remain that way for the entire first five weeks of the period. When we get ready to open the poll for voting for the winner of the contest, we will set it as a sticky topic, and un-sticky the previous one. How does that sound for a more organized system?