How can I stop people stealing my photos?

[Jonathan]

I work in restaurants and during my work I take pictured of all my dishes as a record and to help show what I can produce to help future job prospects. I store all my photos on social media sites because it's free storage and it's nice to have people comment on them. Recently I've noticed that 2 previous employers have taken my photos and copied then to their internet sites. I don't mind but nobody ever asked me and I don't really want to help a previous employer when they havnt helped me. Do I have any rights to these pictures? They're all mine and they've all come from my camera but I don't know how to stop people using them.