Assuming the graphic picker will be a icon with dropdown menu or an icon either way, putting it in front of the Token might be useful. It's consistent with how it's displayed in the menu on the left, and more importantly it's right next to the name (STANDARD, CHILD, MINER, etc), which makes it easy to see at first glance.
If my assumption is wrong, and the graphic picker does not actually display the graphic, I think it's fine anywhere. But the icon should be next to the name.
It'll actually use the exact same UI that the tile picker uses when selecting which tile a creature will use in the token grid. The only reason I hesitate to put it on the left is because you need to select which image sheet the graphic is drawn from before you can pick the specific graphic. It feels strange having to do the fields out of order.
I'm having a few issues with the control library allowing me to do the drawing in the way I want though. The controls are designed to only display text, and I have to override the behavior if I want to pain images in the cells. Everything works fine when the cell with the image does not have focus, but when it does the "text" value of the cell is getting painted over top of the image, which looks really bad. If I can't figure out a way to fix the drawing, I may need to seperate the field in to 2 separate fields, one with just the image, and the other with the drop down to select it.
Also, I've had to change the grouping from the original screenshot already. Now that I'm painting the images it just doesn't work. I'm going to experiment with a few other options.
Thank you for the feedback though, that's exactly why I post the previews so early. I like getting input before I go to far with the coding and feel locked in by the amount of work that I would have to do to change things around.