Question: I would find it more interesting/organized if there was a theme to follow for the featured images each week. For example, this week post only pictures of cars, flowers, landscape or whatever and change the theme every week.

Answer: 
Hmmmm... It seems like that could be something to consider doing when our group is larger, but right now it's not as though we're getting so many entries that it is overwhelming. My concern in adding a theme is that we'd cut the number of entries in half if a theme were introduced. 

There's also the notion of who comes up with the themes? Would the mods be in charge of coming up with something, or would someone need to maintain a weekly thread for voting on the theme like we already do for the picture? Sounds like adding quite a bit more admin work to something that really works fine as it is...
Yes! This is a good idea. It would add some interest and focus to the pictures of the week contest.