The thread seems to have got side tracked with corrections and whatnot, so, back to the op...
"Personally, I wouldn't post a pic of someone elses DC on the internet with out their permission - just out of politeness.
I've quickly Googled, but still not sure about the law re posting images of children on line.
What would be considered an indecent image, what if I'd not given permission for my DC to have their photo taken at school, and another child takes a photo in school/at school disco/ on school trip and posts it on line.
What if a photo were taken on a school trip of my child naked in the shower, and it were posted on line?
Now, this hasn't happened, and I may have an over active imagination, but but what if it did happen?"
If it's not an image of your child in the shower, you can complain to the web site and they should remove it, failing that try Google support.google.com/legal/answer/3110420?hl=en, you'd file a privacy complaint support.google.com/policies/troubleshooter/2990837?rd=2, they would moderate the image and take things from there.
As I said up there, indecent images are graded en.wikipedia.org/wiki/COPINE_scale. The CPS have a lot of information about this but it's very heavy and goes into sentencing etc. A naked child in a shower would be one of these grades, IIRC, so it would need to be removed.
If it did happen, your first port of call is to the Internet Watch Foundation www.iwf.org.uk. If you see an image of an unknown child, then this needs to be referred to CEOPS who concentrate on trying to find the child and making them safe (the IWF concentrate on removing material from the web). The most important thing to remember here is not to forward the image to anyone. Not your mum, your DH or anyone else. Get in touch with IWF/CEOPS and they will tell you what to do. If they request the URL, then send it only to them.
I'd imagine you'd have to consent to your child being photographed if they were at school or on school premises, but should this happen your first port of call is the web site. Then grumble at the school for the lack of supervision 