In my party I have a warlock who is partial to illusion magic. At second level, he took the 'Misty Visions' invocation, with the intent of using Silent Image to keep the party shrouded in illusory fog during combat. His logic is that because the party knows it is an illusion, they are never hindered by it, but the enemies are effectively blinded to the players until they manage to discern the illusion. In one instance, this resulted in two party members swinging with advantage on a bugbear who couldn't see them, and who had to use his action on the next turn to inspect the illusion.
In this situation, Silent Image gives most of the benefits of Darkness, a second level spell, without negatively affecting the party, and without costing a spell slot. I don't want to have to tell the player "No, that's too OP," but I'm not sure how to interpret the mechanics in a way that isn't broken.
What does RAW say about using Silent Image on top of or between close quarters combatants?
How can Silent Image be interpreted/altered such that it can provide some benefit in combat, while maintaining the intended power level?