text popup when you hover over img src ?

sorry to have to ask for help again but this latest thing has me beat at the moment. So I have on my site a image of a phone and what I am trying to achieve is that when a mouse hovers over the picture of a phone that a box pops up saying "call with SKYPE" - I have the <a href="skype:+353*******"> before the img src so thats oK and its doing what I want it to do but I cannot see how to get the text to come up when mouse hovers over the img - is it something like mouseover code in html? - if so whats the correct . have tried the w3 schools website but still confused is there an easier way?
thanks.

edit: - I know I can get a SKYPE button, however i dont want to use the skype icon I want to use my own 'phone' image , plus I dont want it to contact my skype username but for a mobile number instead

Heres the code I am using at the moment : <a href="skype:+35387****"><img src="wpde14f34c_05_06.jpg" width=
"34" height="34" border="0" alt="" style="position:absolute;left:929px;top:236px;" /></a>