I believe that I've found the answer--or at least, an answer.
In case it helps others who might want to do something similar, here follows the skeleton of the code that I used:
/*
Note the use of "nth-child"; this allows us to select the relevant image for each collection.
Within this "nth-child", we want to alter the appearance of the title-link given for
the related collection. This link is produced by an "<a>"-block, which is
contained within an "<h2>" block, which in turn is contained within a div
of class "inner_column".
*/
.collections .collection_row:nth-child(1) .inner_column h2 a
{
/*
The below does three things, if I recall correctly:
1) Force the link to have a given width and height, specifically
that of the intended image.
2) Force the text off-screen (note "text-indent" here)
And 3) Have the link display the intended image.
*/
display:block;
text-indent:100%;
white-space:nowrap;
overflow:hidden;
width:[YOUR IMAGE-WIDTH IN PIXELS];
height:[YOUR IMAGE-HEIGHT IN PIXELS];
background-image:url('[THE URL TO YOUR IMAGE]');
}
/* Repeat for the remaining collections... */