Filing this on behalf of one of our customers:

My thought is it would be nice to have a master image repository that is independent of projects or issues. Currently I am only aware of being able to attach images to an issue within a project. This works, but I can imagine including some standard images in descriptions for recurring issues that I'd like to be able to reference from a global image repository.

Support has outlined to this user how they can use the /atlassian-jira/image folder to save images, then reference these by using the format:

http://jiraserver/images/blah.png

And render this image by using exclamation points on either side, but the user would like a nice GUI for this.