I started using PostImage about three years ago. If I upload a 1920x1080px, and say "Do not resize" it puts it in the gallery at full size, but when you look at the thumbnails (created by PostImage) for that gallery, then click on a thumbnail, the first thing you get is an image 1290 wide, with a link below for the full-size pic. So, if I am putting up screenies, I use IrfanView to resize/resharpen to 1290 wide, and upload that. For comp entries, I resize to 1280 wide. For those pics which really need the full-screen view, I upload a 1290 wide which appears in my post as a teaser, and its 1920 version which is in my post as a link, for people to view if they want to. I don't want to waste their bandwidth.
I think the Screenie Comp limit should be increased to 1290px wide, just to fit in with what PostImage will do anyway. One download at 1290 wide is better than one at 1280, which is then expanded to 1290, in my less-than-humble opinion