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: 
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.)
Can I suggest a sports theme soon? I have not seen one yet, and have searched in the suggestiions.

I am fairly new to this site so do excuse if there has been a few already as I have searched but I do not want to manualy go through the archives of every week.