The devshot camera system is a useful tool for mappers to document progress automatically without having to take screenshots manually every time they make a change. They don't work properly in Alien Swarm however as all images will just display the briefing screen with team selection rather than the level content itself.
This is fixable by modifying the compile options under the 'Advanced' setting.
-makedevshots -hijack -allowdebug -game $gamedir +map $file +sv_lan 1 +sv_cheats 1 +r_drawvgui 0 +asw_cam_marine_shift_enable 0
Breakdown on what each does:
- -makedevshots tells the engine to create screenshots
- +sv_cheats 1 turn on cheats
- +r_drawvgui 0 turn off overlays -- this is the briefing screen
- +asw_cam_marine_shift_enable 0 disables mouse movement changing the screen position and thus affecting the screenshots perspective.
The cameras Pitch/Yaw/Roll will still be ignored and the games top-down angled view will be maintained in your screenshots. So place them accordingly. Personally I prefer to keep the angle as the game default as that is what people will see when playing it versus a 1st person view from the ground.
Information about the Devshots camera system can be found on the Valve Dev Wiki.