Some people are not dumb, but they will search too far something that is achieved simply.
To take a screenshot in game, locate the print screen key on your keyboard, most of time, not far from the numeric pad, and when you are playing a game, press it. A little sound will be played if the screenshot is taken.
If you use a laptop without numeric pad, the print screen command is integrated as an alternative function on another key on your keyboard, just look at all the keys to find it.
In The Conquerors, screenshots are stored in the aoe2/screenshots subfolder. For Forgotten Empires, folder is aoe2/Games/Forgotten Empires/Screenshots. In Age of Kings it's created directly in the aoe2 main folder.
Scroll down to the bottom of your screenshots folder, you should see bmp or png files, your screenshots previously taken.
wide resolution screenshot
Another thing to know, it is possible to take a wide resolution screenshot of the current map.
Press CTRL + F12 to create this type of screenshot (if in a multiplayer game, you must wait the end to use this keyboard shortcut).
This will let you choose between 1:1 and 1:9 map resolution (the 1:1 screenshots are very large, the 1:9 are low resolution), and will capture all the current units and buildings actual positions.
This is the AOC DOTA map rendered as wide resolution screenshot.
Note that this wide screenshot trick does not work with Age of Kings, but a workaround would be to record the game, and open the recorded game file with Forgotten Empires, you should then be able to take wide screenshots from your Age of Kings recorded game pressing CTRL + F12.
Take pictures of the game and host them on some free image hosting service, like tinypic.com and show everyone your games. You can take a screenshot on achievements screen as well.