Similar to Laurie, I often can't find the grab I'm looking for by looking at the thumbnails.
A thumbnail size slider, like in the Mac Photos app, so I can see larger view of each.
When I enlarge the Skitch window, make the thumbnails larger, like in the Mac Finder
Make it possible to drag photos from the thumbnail view, rather than having to go into each one.
Add thumbnail row of 3 previous, 3 next at top or bottom of full image view, so I can keep working without having to pogo-stick from thumnails to full view.
Thank you for listening! I use Skitch many many times/day--it's terrific.