So…
I just had a third coffee and found a site that helped me figure it out.
I placed the following code in the stylesheet:
.fade {
opacity: 0.25;
transition: opacity .5s ease-in-out;
-moz-transition: opacity .5s ease-in-out;
-webkit-transition: opacity .5s ease-in-out;
}
.fade:hover {
opacity: 1;
}
And the following code in the footer.php:
<img src="myimageurl.jpg" alt="" class="fade" >
And it worked a treat. Thanks to for the well-placed tutorial.
Hooray!