WordPress plugin code

I'm currently working on a WordPress plugin, I have most of the code, but the part I'm having

problems with is this:

I'm trying to get it to insert the URL variable into the post by pressing a button, so when you press something such as "Insert", it will add <img src="url"> into the post. If anyone could help, I'd appreciate it :)

I'm also looking for the WordPress hook for adding a button to the posting menu.