I'm using Photoshop, but GIMP should be capable of this. Put the two images into separate layers. Change the opacity of the top layer. You may have to spend time aligning them so that they match exactly. It simulates fading multiple light sources in and out.
I wasn't able to find a way to change the opacity. Are you familiar with Gimp? Could you tell me where to find the opacity adjustments in Gimp?
Much better, I like the photo on the left. I think that if photos are too large, they exaggerate the imperfections on the coin and cause the viewer to grade the coin lower than it actually is.
Yeah, that makes sense. To me it was the other way around though. When I was going back and forth between the two it was before I edited them. The coin looked smaller because I hadn't cropped it yet. Once I had the coins cropped, enlarged, and in this thread I almost immediately gravitated towards the first one, which is the one on the left in the side-by-side.
I have only dabbled with GIMP. I found this tutorial on blending two images with different exposures. It shows opacity as a slider above the list of layers.