At the link it notes, "During the years 1997 to 2004 the United States Post Office produced nearly 1.75 billion postage stamps that contain a "hidden image". These covert designs are only visible when viewed through a special device called the "Stamp Decoder."
"The Postal Service's STAMP DECODER is a decoder lens manufactured especially for reading hidden images
and symbols on selected stamp issues. The decoder lens is a precision optical device manufactured with the
same acrylics used for prescription eyeglass lenses. Each decoder is comprised of nearly a thousand tiny
elongated lenses called lenticules."
"Scrambled Indicia® is Graphic Security Systems Corporation's patented pre-press process that scrambles, distorts, intertwines, overlaps or otherwise manipulates images making encoded information on them unreadable by the naked eye, and non-copyable by current color copiers and digital scanners. When printed, the images are decoded through a simple decoder lens allowing the viewer to see the encoded images."
Listing of U.S. Stamps With Hidden Images (Sorted by Scott #)
Scott# - Date - Face Value - Stamp Name - Hidden Image
3036 1998 $1.00 Red Fox, Hidden image: Fox
3167 1997 32¢ Air Force Thunderbirds, Hidden Image: “USAF", Star in circle, View at 15 degrees
3168 1997 32¢ Lon Chaney as the Phantom of the Opera, Hidden Image: 2 Opera masks
3169 1997 32¢ Bella Lugosi as Dracula, Hidden Image: 3 Bats
3170 1997 32¢ Boris Karloff as Frankenstein, Hidden Image: 3 Lightning bolts
3171 1997 32¢ Boris Karloff as the Mummy, Hidden Image: 2 Egyptian deities
3172 1997 32¢ Lon Chaney, Jr. as the Wolfman, Hidden Image: 2 Howling wolves
3178 1997 $3.00 Mars Pathfinder, Hidden Image: “•MARS PATHFINDER•JULY 4,1997", "•USPS" (repeating)
3206 1998 32¢ Wisconsin, Hidden Image: Badger
3230 1998 32¢ Bright Eyes Dog, Hidden Image: Dog bone and dog house
3231 1998 32¢ Bright Eyes Fish, Hidden Image: 8 Bubbles
3232 1998 32¢ Bright Eyes Cat, Hidden Image: Paw print and mouse
3233 1998 32¢ Bright Eyes Parakeet, Hidden Image: Bird cage
3234 1998 32¢ Bright Eyes Hamster, Hidden Image: Exercise wheel
3238 1998 32¢ Space City, Hidden Image: Large & small spacecraft, figure
3239 1998 32¢ Spaceship landing, Hidden Image: Small spacecraft
3240 1998 32¢ Person in space suit, Hidden Image: Small spacecraft
3241 1998 32¢ Space ship taking off, Hidden Image: Small spacecraft
3242 1998 32¢ Large domed structure, Hidden Image: Large & small spacecraft
3261 1998 $3.20 Shuttle Landing, Hidden Image: "ENTERPRISE/COLUMBIA/CHALLENGER/ATLANTIS/ENDEAVOR/DISCOVERY"
3262 1998 $11.75 Shuttle Piggyback, Hidden Image: "ENTERPRISE/COLUMBIA/CHALLENGER/ATLANTIS/ENDEAVOR/DISCOVERY"
3321 1999 33¢ Skateboarding, Hidden Image: “GNARLY”
3322 1999 33¢ BMX biking, Hidden Image: “RAD”
3323 1999 33¢ Snowboarding, Hidden Image: “SWEET”
3324 1999 33¢ Inline skating, Hidden Image: “PHAT”
3472 2001 $3.50 US Capitol, Hidden Image: “PRIORITY MAIL”
3473 2001 $12.25 Washington Monument, Hidden Image: “EXPRESS MAIL"
3647 2002 $3.85 Jefferson Memorial, Hidden Image: Jefferson’s Signature
3648 2002 $13.65 U.S. Capitol Dome, Hidden Image: U.S. Flag
3651 2002 37¢ Harry Houdini, Hidden Image: 4 Chains wrapped around Houdini's body
3771 2003 80¢ Special Olympics Special Olympics logo, View at 90 degrees
3787 2003 37¢ Old Cape Henry, VA, Hidden Image: Date: 1792
3788 2003 37¢ Cape Lookout, NC, Hidden Image: Date: 1859
3789 2003 37¢ Morris Island, SC, Hidden Image: Date: 1876
3790 2003 37¢ Tybee Island, GA, Hidden Image: Date: 1867
3791 2003 37¢ Hillsboro Inlet, FL, Hidden Image: Date: 1907
3808 2003 37¢ Bronko Nagurski, Hidden Image: Football Player
3809 2003 37¢ Ernie Nevers, Hidden Image: Football Player
3810 2003 37¢ Walter Camp, Hidden Image: Football Player
3811 2003 37¢ Harold "Red" Grange, Hidden Image: Football Player
3838 2004 37¢ U.S. Air Force Academy, Hidden Image: Peregrine falcon, View at 90 degrees
3862 2004 37¢ National WWII Memorial, Hidden Image: U.S. Flag