We wanted to remind you about this weekend’s upcoming photo contest sponsored by DCist and non-profit Cultural Tourism DC as part of the semi-annual WalkingTown DC event. On Sept. 29, photographers are encouraged to attend any of the free walking tours on offer as part of WalkingTown DC, and enter their photos to win great prizes. Details and rules below, and also feel free to sign up for the unofficial Flickr group. Here’s the deal:…