Can we change the in-game opacité to make effects ? (like clouds, light or dusts)
(for game maker it would be: https://docs.yoyogames.com/source/dadiospice/002_reference/objects%20and%20instances/instances/instance%20properties/image_alpha.html )

I would think so seeing how the 3rd rule is "you can use other tools create transparency." At first I thought that meant to remove the white background, but I'm thinking about it now, it should cover opacity changes. The host of this jam should clarify though before you go and prepare a game that will require the alpha change.

You can totally use opacity effects in game! Most of the rules are based around asset creation, so don't hold back what you do in the actual game engine! :)