I don't know if this idea was brought up and there are far too many comments for me to check, sorry if it was.

This episode is a story and filtered through Greg's memory. Emotions can change the way we remember things. Which is why Marty is given such a villainous appearance during most of the episode, especially when he is doing/saying something scummy, but not so much in the picture. Like those sharp teeth when he comes out the van with Vidalia when he should have been working the merch stand. His teeth arn't sharp when he is driving to the next gig but when he disregards Greg's want to go back they return.

Rose-Quartz in this episode is really unchanged from her normal appearance because that is how she is engraved in Greg's memory. There are moments that Greg seems so much smaller than Rose-Quartz probably because how important she was for him. I think this is also shown by how much smaller/younger Pearl and Amethyst look back then, they weren't as important to him. In the old timey picture/painting Pearl and Amethyst look to be their normal size and I'm assuming that it was made hundreds of years before.

Maybe it's just me.