MyBB Community Forums

when posting insert an image where can I find the code that makes this work

This is quite a complicate task, do you want to keep the image after the user edits the post? Anyways, I'm sure all hooks you need can be located in the mybb/inc/datahandlers/post.php file.

If you are asking for a out-of-the-box solution, I don't know if there is any.

can you elaborate your requirement ..

do you want displaying a static image with every post ?
or is it like a place holder ? or is it for showing information ?

see also default message plugin