While you're figuring out what to do about your old style sheet, find the style definition for IMG that sets a border of something other than zero and change it to zero. If there is no definition, add one that says...
img { border: 0; }
What's happening is that the default browser behaviour is showing through. Browsers will draw blue borders around images which are links (just like they make linked text blue unless you tell them otherwise). And if it's a link you've already visited, it'll be a purple box (or purple text).
So set the border to zero and the boxes go away.
Rob
PS
Pay attention to what Kobra says. He da man!