For future reference: (And because its quite comical)
If you edit a post by changing an image the old image still resides as an 'attachment'....although no longer 'in' the post. [inline]
To remove it you need to click the 'Go Advanced' button.
When you do the page will reload with your 'attachments' displayed at the bottom.....both the new and the old.
You then need to click on 'Manage Attachments'.
When you do you get a window open with all the images you have uploaded to date that are still used in posts.
At the very bottom of that window will be the thumbnails of the images you have used in the post you are editing.....both the new and the old.
You need to delete the 'old' image....hover over the thumbnail you want to delete and click the 'X' when it appears.
The thumbnail should then be removed.....leaving just the one you want.
[ It can be extremely difficult to know which thumbnail to remove, especially if they are similar....so be careful...! ]
Then click on 'Done' bottom right of the window.
When you return to your post it should refresh and the 'old' attachment should be removed....leaving just your new one.
You can 'preview' your post from here or just save it if you feel confident enough.
I don't know why you had trouble with it.....
Regards.
MrToM.