I just loaded up Pools of Darkness in EGA mode. Some of the artwork is fine (like the picture of Elminster below), and some of it is hideous (the other picture...what is up with those eyes?). So, I'm guessing at the point that this game was made, they put less care into how the EGA artwork was handled. They didn't even get Elminster's colors right, so I don't think anyone even looked them.
Considering these look worse, I'm going to guess they didn't start as EGA pictures and actually began as VGA. I think the opposite is true of the earlier games that had VGA modes, but that's just a guess. Not sure if that helps in any way, though (probably doesn't). But, I thought was interesting.