If your thumbail image doesn’t reflect the design as you would like, there are a couple of ways to change it to something more useful.
Make a copy in the workspace, make it HUGE, then move it just outside the printable area. My passthrough slot hold-down pins are down on the bed, but I put a couple of massive copies above the printable area, and the thumbnail shows those nicely:
Another is to take the same idea, but use text as big label:
You could use any image, import and scale up, to represent your design - like an assembled picture.
Hope this helps!