Adding a simple new option to the toolbar

Hello, I'm trying to understand how to add a new function to the toolbar-set.
I want to add a button called "Add formula", when the user clicks on it he will be prompt for the formula (a simple javascript prompt will do fine) and the editor will add a <img src="http://someAddress.com?formula=HIS-FORMULA" /> to the text.
can anyone help me please?
thanks in advance.