Screenshots saving as 640 x 480 pixels

[jasonhex]

Hello, I'm looking for some tips to increase the screenshot size to match the resolution that I'm actually seeing. My native resolution is 1920 x 1080, and when playing the game (using zdoom) the game's full screen fits nicely and looks great, but the screenshots are small, fat and ugly! How can I fix this?

Using the Steam overlay to take screenshots (F12) and using the Print Screen button in-game result in the same ugly screenshot. Any help would be greatly appreciated!