I expect you are on here asking about potential problems because you suspect there may be... I would go with that instinct..,
1) Context .... Also important .... Taking a pic of your mate with a stranger in the background that you post to FB or put in a book, probably not a problem... Taking a pic of a stranger that they later discover is being shared around social media, potentially a problem, particularly in context of how that image is being used....
2) The greater the exposure, the greater the risk of those images being seen by someone who may take issue with it being used without their knowledge or consent
3) You don't necessarily have to do something illegal to get sued