–2nd EDIT–
Please note that this answer doesn’t work any more and is out of date. Please try the above answer.
— ORIGINAL ANSWER–
Got it sorted. Had to use a url that is chock full of craziness:
<a href="http://www.facebook.com/sharer.php?s= 100&pFacebook Share doesn't show my description or my thumbnail=La Vita È Bella&p[url]=http://www.lavitaebella.co.uk/teachers/resources.html&p[images][0]=http://www.lavitaebella.co.uk/images/imageforfacebook.gif&p[summary]=Drawing on Italy’s most famous export – great-tasting, healthy, colourful food – La Vita é Bella brings families together to experience mealtimes the Italian way"><img src="../images/share/s-fb.png" /></a>
–EDIT–
As some extra information, I can’t remember where I originally found this information but here’s what the s=100 part means:
The basic approach is you are providing a link to http://www.facebook.com/sharer.php?s=100 which is the basic share page (typically this is shown in a popup).
But I tried changing the numbers and it still goes to the same place.
As for the &p, it’s just html code for &p and I think this is the array that Facebook dissects to get at the information.