With Chris's inspiring post here:
https://css-tricks.com/different-transitions-for-hover-on-hover-off/
I managed to come up with this:
#banner
{
display:block;
width:100%;
background-repeat:no-repeat;
background-position:center bottom;
background-image:url(../images/image1.jpg);
/* HOVER OFF */
@include transition(background-image 0.5s ease-in-out);
&:hover
{
background-image:url(../images/image2.jpg);
/* HOVER ON */
@include transition(background-image 0.5s ease-in-out);
}
}