Source: Steampowered Forums

First of all you have to take your screenshots using the IMPR PANT key on your keyboard or any screen capture program, for example FRAPS.

Remember: screenshots must be in JPG format, otherwise Steam won't recognise them.

These are the steps to follow once you have your screenshots ready:

• Go to Steam/userdata/YOUR NUMBER/760/remote/243380/screenshots.
  If the folder 243380 is not created you have to make one with the same name, and a folder named screenshots inside. Thumbnails folder is not needed, because Steam generates one itself.
  
  
• Now put your images inside the folder screenshots with a sequencial name like
  "2013-12-22_00001.jpg" following the last number already named.
  
  
• Now restart your Steam Client, open the Screenshots Manager and upload your Might & Magic VI screenshots to Steam Cloud.

This can be done with any game, just find the GAME-ID of the game by making a desktop shorcut, then right click on it and properties. You will find the ID after "steam://rungameid/"