It would be wonderful if we could upload revised or replacement images so they just replace the earlier version wherever it appears. Maybe an (update with new version) button in the media library. This seems to be a common problem - wikispaces.com also has the same issue.
Having inserted a large number of images into my blog I realised that they all needed editing, so I had to go through and delete them all from the library and the posts, upload the new versions, insert them again, re-edit that picture data, it was a real pain.
Greg Maletic posted a help request about this last year <http://en.forums.wordpress.com/topic/how-to-change-already-uploaded-images?replies=7#post-177291> but everybody seemed to miss his point.
Best wishes from Sheffield
The blog I need help with is sevenleagueboots.wordpress.com.