nope if the image is on your PC its your problem... the site (i believe) sent conformation the girls were over age via their legal rep to the court, however this was not proof enough and he had to hire a PI to track them down and pay 2k for each cert... + fees etc etc...
Wasn;t there another case recently where the guy managed to get copies of every girls birth certificate proving they were over 18 except for one and he got convicted as she looked underage and put on the sex offenders register?
AFIK, it doesn't matter how or why the image is on your hard drive or even in the cache, you have to prove they are not underage.