So I've worked out what ISN'T wrong. I've tried the Firebug add on. Open page in question, clicked firebug icon in top-right of browser, hovered over where the image SHOULD be, and did NOT get the said thumbnail.
I've tried using the embedded style sheet. Code is working, seeing as the text and everything else, borders and colors have changed as per the code.
I've also double checked the file path, by putting the image straight into the html on the page. And all appears well and good. EXCEPT, the image is still not showing up!
I have had an idea. The image I'm using is white fading to a photo. And it struck me that maybe something WAS wrong with the SIZE of the image. Could the size of the image be so large that it stretches beyond the borders of the div, so that only the white portion is visible (or invisible, in this case) ?
And what would I do with the css to adjust the image to fit just inside the div?