Custom image buttons with different hover over image

I’m very new to WordPress and have found answers to almost all of my queries by a simple search so I might be searching for the wrong thing when I’m looking for an answer to this as I’ve struggled to find this in a topic so far.

I am using the Responsive Theme (excellently designed may I add & impressive support from what I’ve seen in these forums) in WordPress 3.5.2 and the site is currently on a temp address;-
https://173.254.28.144/~horizoq5/
due to be transferred shortly to;-
https://www.horizonpcs.co.uk

I need some help in using custom built images as buttons where I have three image states – an up, hover over and down (on the homepage of this link it is the ‘Contact Us’ image in the bottom left widget)

I have pulled in an image to a text widget and I was hoping to simply add some more text to this widget so the site would look at the other images if the mouse hovered over or clicked on the button.

<a title="Contact Us"
href="https://173.254.28.144/~horizoq5/contact-us/"><img class="aligncenter"  src="https://173.254.28.144/~horizoq5/wp-content/uploads/2013/06/Contact-ButtonUP.jpg" width=272 height=150 /></a>

I have been able to create a child theme but if this has to be done on the style sheet or a php doc could you please advise in simple terms for a complete novice.

Appreciate any support, thank in advance!