Is it illegal to take a picture of someones dog and post it?

i don't think it is illegal but as a common courtesy you should ask ... i was super ticked off when my groomer used pictures of my bichon without my approval ... and i work as a dog walker/sitter and on my website i have pictures of all the dogs i have walked and/or sat and i asked permission from every single owner before i did so ... i don't think it would be awkward asking the owners at all ... everyone i asked said yes and even liked the idea of their dog being shown off ...