Post screenshots at the resolution you were playing at when they were taken. e.g. if you play at 1920x1080 post at that resolution, 2560x1440 post at 2560x1440, etc... No thumbnails or 500 wide (etc..) pics.
Downsampled pics to be posted at the screen's native resolution. e.g. 2880x1620 on a 1920x1080 display, resize the 2880x1620 screenshot in a photo editor to 1920x1080 using bicubic, Lanczos3, supersampling, etc...
X amount of shots per post, Y amount of consecutive posts to prevent overposting.
Posts should include game title, preferably along with any mods (unless it's a huge list) as well as any IQ tweaks (SweetFX, SMAA, SGSSAA, 16x AF, etc...). The more info on how a shot was achieved the better.
No overcompressed jpgs (this means no Steam shots). I've taken to setting FRAPS or Dxtroy to autoshoot every second and then I convert the bmp to a 97% jpg with no subsampling and it's indistinguishable from a png at 1/3 the size.
I would also up videos to maybe 4 a day seeing as so few people post videos. I could also be roped into making a quick and dirty Dxtroy/Lagarith/RawCap/Handbrake preset/Youtube guide so as to get more people capping and uploading.
Again all suggestions, but I think everyone would benefit from increased quality of posts. Not that they aren't quality already, but there is still room to become even better.
And just to explain the jpeg subsampling thing, left is no subsampling, right is normal jpeg subsampling.
I used an image with no antialiasing just so the difference would be clear. You can also see the loss of color in the top right corner where the red pickups become rather muted.