A photo bomb is one thing. But, aiming for another in a picture is an image capture. So, ask for the photo for picture networks. Given, proper netiquette get permission.
Ask Netiquette Picture Networks
Mobile devices allow people to take group photographs easily. Sometimes members of groups are not aware that they are being photographed because it happens so fast.
Even if your using a regular camera asking permission before you take a picture is good netiquette because consent is required to upload photographs to a picture network.
Picture Networks Ask
Subjects should be informed that the picture will appear in social media. This allows them to look their best and take good photographs.
A subject might not care if the photo is not going on the internet. Knowing that it will be uploaded may change the posture of some of the members of the group. Others may opt out of the picture for various reasons. That is their right.
It is proper netiquette to ask permission to take group photographs because it makes everyone aware.