Why not just keep your project files with their images? Spine can use relative paths.
If you open the Spineboy.spine project example, you'll see that its images path is ./images/
What some people do is keep their Spine projects with their images in a subfolder and keep them in a shared Dropbox folder. It works fine. OSX or Windows.
You could also just copy that entire folder and send it to someone. That would work fine too.