Should have looked on sitepoint first as I've answered this question hundreds of times in the css forum
Hmm that article doesn't seem to adress the correct reason (unless I missed it when I read quickly through it ).
Images are treated the same as text by some browsers (and depending on doctype mode etc). What this means is that an image will sit on the baseline (not the bottom) exactly the same as text does and will allow room underneath for descenders in the text.
Setting the image to display block will solve the problem or setting the image to vertical-align:bottom if other content is n the cell and display:block can't be used.
Hope that helps.