Yes it would need to be an extension because it needs to have the topic_id the image goes with.
The image url should be local images also in my opinion for a few reasons.
Topic images can be uploaded to the server. I like using Image Upload
by dmzx and martin.
I've set it up in posting for adding the image like this.
Top image is creating a new topic. Bottom image is editing first post of any topic to add or change the image.
I need to work up the event/listener.php to get all the functions set. How would this setup work?
Of course off site image urls can be used also but you risk if that image will always be there.