The cute critters are digitally manipulated by
Steve Bicknell of Icarus Arts Publishing. You can buy the 2009 Calendar featuring the guinea pigs at Maverick Arts Club.
Life in the Fast Lanes has a funny account of the guinea pigs:
The Guinea Pig Games (GPG) Olympics have started with an amazing opening as rascally rodents display their dedication to their chosen sports. In the struggle for survival, only the fittest win out at the expense of their rivals with their stamina, prowess, and incredible feats to win gold, silver, and bronze medals.