Here's what I would do -- DELETE the message with the wrong picture, then post a brand new message with the right picture attached to it.

How to do: Open the message. Since YOU created it, you can delete it too.
Look right above where it says "Message in the Thread". There's a line right above it that says "Manage Message" with a drop-down box beside it. Click the box. One of the choices is "DELETE". Choose it, then fill in your password. Click the arrow, and you have deleted the post. It can't be read anymore.

Then post a brand new message as you normally do, with the correct picture.
Also, if you PREVIEW the message before posting it, you can see the title/message/picture before posting. And you can edit/correct it there, if necessary, before you click "POST".