Warum werden 'img'-tags in 'td'-Umgebungen immer mit bottom margin dargestellt? Die Anwort auf 'stackoverflow':
This problem is caused by the image behaving like a character of text (and so leaving a space below it where the hanging part of a "y" or "g" would go),
and is solved by using the vertical-align CSS property to indicate that no such space is needed.
Almost any value of vertical-align will do; I'm fond of middle, personally.