Actually, I have heard a bit where it was not the Google car that took the photo, but someone who uploaded the photo to Google.
If that is the case, then Google might need to scrutinize incoming photos a bit more, but really, we're still talking about millions. I'm sure the people who do this have brains of Tapioca after sifting through photos for 6-8 hours a day.
Either that or people should do a bit of self checking to alert Google to pics in their neighborhood that might be inappropriate.
Heck, have alerts sent to you if new pictures get posted about a region you have interest in (ie home) if you want to keep an eye for these things? Bet that would be nicer.