The problem with a thread like that is it would need to be a sticky. The people who can't be bothered to disable voting on images that clearly have nothing to vote on would likely ignore it. Nothing really to do but tag it "disablevote" or something like that and let Kid_Kyoto take care of the rest. However there are some great
WIP images out there sometimes showing off a nice sculpt someone has done or some cool conversions, for that its vote N/A for the painting and just vote on the coolness.
It usually ends up on the preference of the voter most probably have their own. Some may vote down because it's a faction/colour they don't like or vote up if they do. Some people even vote on their own images, maybe it's just for them to reflect on how well they thought it went, maybe it's to boost the score.
For me Table top quality is 4. This way I reserve the 10 for the very best of the best. An example of a 10/10 for me is this.
Don't be worried about voting though, most are grateful for any votes on their images (I know I am

) as it helps them get an idea of what level they're at and if they're ready to go out and try some new techniques they may have though of as too hard for them before. Comments also help out that way as well.
In the same way I save 1/10 for the absolute worst, the ones that look like they were thrown at a bucket of paint of left to hibernate a year in one. I think I may have one or two votes that low as like the 10/10 it takes that special something extra to reach it.
I'm by no means a harsh marker, I just like to leave something extra for the truly great stuff. If there was an 11/10 I'd use it for the same thing.