The only thing I’ve ever seen in that stacks area are images that come delivered with individual stacks (mostly older stacks). I have never had an image that I put in, show up in that panel.
If you drag an image into a stack, it should get published without any problem. I think that is a legacy library left over from an older version of stacks(I started with version 3), Like the template library.
You can use resources, but as the post above talks about it is not drag and drop, and a lot of folks have had issues using it.
Many users have gone to managing their images themselves by exporting them to the web server and referencing the URL from within RapidWeaver(aka warehousing).
This book explains that process: