Story behind this screenshot:
CI3 was (and still remains to this day) the most technically revolutionary episode in the series. There was a jump from strictly rectangular 2D blitted bitmaps to fully blended/rotated/scaled/tinted/deformed 3D textures, an increase in resolution from 640x480 to 800x600, a switch to fully orchestral soundtrack and, perhaps most impressively, the capability of playing multiplayer games over LAN or internet. This was a hot feature and not something that a lot of games offered back in 2006, so the screenshots leaned heavily towards showcasing it (hence the 4-player screenshot above).
CI3 was initially only available on the IA website, but the next few years saw the rise of aggregator/publisher websites like Real Arcade, Game House, Big Fish Games etc., and it wasn’t long before CI3 appeared on those websites as well.
There was a problem, though. The multiplayer functionality wasn’t compatible with those sites for a combination of business reasons (e.g. a no-network-traffic policy on their part) and technical reasons (matchmaking servers could only handle accounts created on the IA website). So, for one reason or another, multiplayer was removed from those versions.
To avoid confusing and misleading players, the art assets supplied to those websites also needed to avoid showing multiplayer. Sometimes this was achieved by using single-player screenshots, and other times through doctoring of existing multiplayer screenshots so as to remove evidence of multiple players (also consider that this was during a time where creating new screenshots was not as straightforward and streamlined as it is today – a story for another day).
And this brings us to the punchline: This screenshot is in fact a two-player game retroactively made to look like a single-player game. The screen contents have been shifted right to hide the second player (whose Plasma Rifle can still barely be seen). The reason the rifle wasn’t completely hidden is (most likely) that the shift needed would make the empty space on the left side too large and unbalanced.
So now you know™.