For instance, if you look closely at the art of Donato Giancola's Chrome Mox, you can see that it has a small figure reflected in the mox's chrome surface. Under magnification you can see that the figure is actually a famous artifact legend from Urza's Saga - Karn, Silver Golem.
Also, the art of Genesis Chamber shows a Myr being shot from a large structure. Not many people besides artist Mark Tedin (and now you) know that along the outer rim on the structure are tiny runes which spell out "COMBO WITH SKULLCLAMP."
Even more interesting, if you zoom in on the art of Black Lotus, you can actually make out the face of what appears to be pop icon Britney Spears! And what's even odder is that although Ms. Spears was only twelve when the card was illustrated, artist Christopher Rush captured the way she would look in the 21st century. Amazing!
Many thanks to image alteration wizard Jen Page.