I have a few suggestions:
- Move the code to GitHub (I could do that for you) and push your versions/tags to wp.org (a free automated service for that is coming soon http://bit.ly/1dDZBXU )
- move the screenshots to the assets folder (less files to track in trunk)
- question: why/how are you changing always the complete file on a new release? That is totally killing the purpose of version control. No one can see what has changed that way.
If you want to go for GitHub and have any questions just ask I might be able to help ;-)