I think you are being unreasonable, sorry.
Whatever the circumstances of your child's adoption a random photograph with your child somewhere in the background - probably not facing the camera, behind a toy, out of focus is not going to be the thing that "outs" your child to his biol. parents. In the wierd circumstance that the bio parent did recognise the child, they still don't know your names, your house, any details of your life. It's really hard to find a photo online without some identifying tag (name, school etc labelled on pic) and the stranger who took the photo won't be able to give them anything like that.
I have heard parents of children that have died always "seeing" children that they imagine look like theirs. Presumably the real bio parents might experience this all the time and be wrong... so even seeing a pic they would most likely assume this was another mistaken identity.
And for this slim chance of being recognised, you are impinging on everyone's behaviour. I agree that noone needs to take a pic, but sometimes it's the first ladder climbed, friends from afar meeting each others kids for the first time - small milestones. Or the photo may be more significant to the person taking it... in my mind it's as likely that this is the last photo of someone's own kid before a tragedy occurs (house fire, car accident, diagnosis of serious illness) than your adopted child being recognised in the background of some stranger's photo.
If your worried about it, move in front of your child, distract them to another area where there is no camera or stay home... but let other people enjoy themselves and take pics of their own kids if they want. For all you know it could be significant to them.
If someone tried to take a closeup shot of your kid, then sure whinge away, but a random shot in your vicinity is unreasonable to whinge about.