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...
I think this is a good idea too. I think @Jay brings up a good point about managing the which weeks have themes and what the themes are. I figure interested parties can hash out a system if we do choose to go with themes. 

Here are a few random ideas (one stolen from @seanmc):

* Unedited (strait from the camera, make it as perfect as you can with nothing but physical equipment)
* Photomanipulated (do something unconventional, add photographic elements and blend together a new reality...or surreality, or something completely unreal!)
* Brilliant Sunsets
* Monstrous Mountains
* Micro Macros
* Pure Portraiture
* Dripples (closeup water shots...ripples, drips, droplets, etc.)
* Birds of a Feather (any shot of birds)
* Wild Life (wildlife..duh!)
* City Life
* Nuanced Night (any shot from nighttime)
* Astrophotography

As an added thought, from a management perspective, it might be good to have a "Theme Announcement" thread a week ahead of time to give people time to go out and get a shot for the upcoming theme (if the next week IS a theme...to @dpollitt's point it might be best not to have every single week be a theme week.)