As general nitpickery, it s/b "onclick" instead of "onClick", and your img tag should be closed with a "/>" insead of ">". I doubt these will solve the problem, but it will make
the W3 Validator like your site better :)
Does Firefox give you an error when you click on the image, or does nothing happen? Is the function supposed to do something every time you invoke it, or are there situations where it might return without doing anything visible? Have you put an alert at the head of funcSearchUsers to verify that it is being invoked properly?
Your tag looks fine. I wonder if the problem is in the context. Post some relevant code around your tag.