Creating Code buttons for forms

Searched the net for this but I'm guessing I've been searching the wrong terms. On my blog which is manually set up (i.e. not wordpress) I have a simple form function to write my post and send it to the blog. I tend to type the html myself dependant on what I want in the article, however does anyone know of a tutorial/explanation of how to add in buttons that will paste code into my input box in much the same way it works on this forum.

Basically so I can make a button that says instert image, I click it and into my input form it puts <img src=""> read for me to just enter the url.

 

Hmm, they look a bit complicated to initialise for what I want to do. The form that posts to my blog is literally a bog standard form, 1 single line text field for subject, one text area for content that I type HTML into. I basically just want to remove the need to type the html, have some buttons that paste html code into the text area for me.