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: 
Yes! This is a good idea. It would add some interest and focus to the pictures of the week contest.
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.