When I was asking about the doctype I was curious about the html standard you are using, if it is xhtml 1.0 then the validations will catch any missing slash like this.
On your update page it seems that you have some formatting applied to the editor. In the configuration for the editor you should ensure that you do not have any formatting present for the initial value. If you cannot spot this post back with the update page and I can point out where this change will need to be made.
Also, when you are inserting this for the first time are you able to confirm in the source code view that the image has the trailing slash?
If you post back with the insert page I can look to see if there is some formatting on this page that would cause this to happen.