Some people go back and change their posts if they are having an argument/discussion with others. On other forums I have seen people change their entire post to a blank space during heated discussions just prior to leaving the site in a huff...
Then there are those who just want to be able to change the photo attached to a recipe to a better one.
In the case of the first instance, then anything you post is public, changing your mind up to a year later when you have a discussion/argument should make you reconsider what you post in the first place. This is a food/cooking forum not a general debate/argument forum (they do exist).
Personally in the second instance (photography) I think if this is the case then either ask the moderators to change it for you, or simply accept the fact that you have gotten better at photography, say so and post the new picture in the reply to show progress and not put off new members with stunning photography from the word 'go' and make it look like that has always been the case.