Not sure if this has been asked yet but I can no longer seem to take the hi res screen shots only the standard 1029x1080 5.9 MB ones now.
How do you take a HI-REs Screenshot?
Normal screeny - F10
Hi Res Screeny - ALT+F10
Are you playing in Open? The hi-res screenshot functionality is only available in Solo (and has beren for some time) as it could be used by cheats to artificially introduce lag.