I believe there is a bug that has been fixed but not released yet where the Images path doesn't show after browsing to a new path. It has changed though, eg if you restart Spine you'll see the new path. You can also type a path in or copy/paste it from a file browser, then press enter.
Spine may have trouble opening some PNGs, eg Adobe includes a bunch of junk in PNGs unless you use Save for Web.