I've run into that problem. Unfortunately I've not been able to get the OS screen capture to deal with it, but instead if I'm capturing from a video, I've had to figure out how to capture frames using the video player. VLC is very good for that. However your problem is beyond that, you have a game playing and wish to capture video of that. Similar problem for embedded video on web sites, they're hard to capture. The difference there is many browsers can have add-ons which allow capture of embedded video contents, Download Helper is one that I've seen and used.
I'd recommend since you mention that you're playing online games; see if one of the video capture add-ons for a browser would capture a video of your play and then from there if you choose to post just a screen shot, you can use something like VLC to grab the exact frame you wish to use.
Any simple methods to set video modes or modify settings to make it so that the screen capture works, have never worked for me. The main issue I think has to do with renditions of frames for video and it's not accessible as a normal screen buffer, for capture purposes that is. You get the replacement "green screen space", which is space reserved for the video rendition, "done by another device" as far as the main screen renderer goes. It was enough of an explanation that I thought "WHATEVER!" and moved on, also because most of the fixes were: complicated, they were a "maybe" answer, and they were specific to certain hardware configurations or chips.