Prepend or append value in the attribute value

Prepend or append value in the attribute value

Say I want to alter the existing value of an attribute.

To change this:
  1. <img src="images/bla.png" alt="" />
Into this:
  1. <img src="images/new_bla.png" alt="" />
How can I get this working with jQuery?
So I don't want to change the whole value of the src attribute, but just prepend a new value inside it.

Anyone?