Ok currently for my site a:hover is set to have a border on the bottom, but I don't want this to be true for images.

Heres the css bit

a:link {
text-decoration: none;
color: #474C51;
}
.text_help {
margin-left:11px;
margin-right:10px;
}

a:visited {
color: #474C51;
text-decoration: none
}

a:hover {
color: #474C51;
border-bottom:dotted 1px #000;
text-decoration: none;
}

a:active {
color: #474C51;
border-color: #A4A8AB
}

img {
text-decoration: none;
border-bottom: none;
}

a:hover img {border: 0 !important}

should fix that :)

a:hover img {border: 0 !important} should fix that :)
Didn't work.

drop the ':hover' part so it only says

a img {border: 0 !important;}

Should work mate!

You have to create a class and apply it to links that contain images.. for instance:
a.noborder:hover { border-bottom: 0; }
<a href="#" class="noborder"><img src="img.gif" /></a>