We discussed the card Clone and its new art in a previous article, but it's worth another look, especially if you weren't around when clone was in print the first time.
Julie Baroh's clone, from Alpha, was an interesting piece rendered in colored pencil that showed two soldiers staring at each other, not sure what to make of one another. Over time, it became one of the more recognizable Magic pieces. Carl Critchlow reiterpreted it (using paint) for the reprint of Clone in Onslaught. Same beach, same clothes, same soldiers, except they now brandish crossbows and look slightly more irritated. A fine homage to the original.
Another well-know piece of art, Morphling by rk post, is similar in appearance to Clone, even though it isn't a "copy" card per se. Morphling was actually designed to be a sort of "rules-friendly" Clone - a card that could simulate other creatures without actually copying them. That's why the art is similar to Clone's.
Two other blue cards use mirror image art: Quinton Hoover's Vesuvan Doppelganger from Alpha (Clone's big sister), and it's mild-mannered descendent, Greg Staples' Shifty Doppelganger from Odyssey.