This can be a bit annoying to do. The current game screen is stored in $game_map, so to access a picture, you have to type this:
i must be between 0 and 20 (though 0 isn't used in the editor).
Now, to tint it, there is the start_tone_change
method. It takes two arguments, the tone and the duration. The tone is really just a Tone object, so you'll have to create it beforehand.
Will make the 1st picture turn grey in 40 frames.