Following a chat with a fellow CMDR in game last night I thought it might be worth posting this in case it's of use to others.
If you find that the grabbing of High Resolution screen shots (Alt+F10) isn't working, check for other applications that might have that keyboard shortcut configured. Example
: nVidia Experience defaults to have Alt+F10 to grab the last 5 minutes of video footage. Change that shortcut (or remove nVidia Experience) and the joys of much larger pictures will be yours once more!