Your own is definitely best, for two reasons. One is that you were there to take it, so you observed the subject and will remember more when you go to paint. Your mind can then correct things like camera distortion or obscured details. The second is that you won't have to track down the photographer for permission or worry about someone accusing you of copying, and perhaps getting your art removed from your site.
Avoid images you find online UNLESS you get permission from the real owner, or the image truly is in the public domain (so old that copyright expired or never existed is one reason it might be in the public domain). There are sites that claim the images they offer are free/public domain but many are offering stolen images, and some of the really bad ones infect viewers' computers. This site is good for dispelling myths as well as helping people find actual public domain images to work from: http://www.publicdomainsherpa.com/10...ic-domain.html
If artists really have to look at online images for reference, they can look at lots of different images for general things like colors, proportions, etc, and not make a close copy of anything. Make your own composition.
Ideally, sketch from LIFE. That's the way you learn and remember things the best of all. Lots of sketches and your own reference photos to finish up back at the studio. I've found many photographers who, if asked, gave me permission, but I still like to observe and sketch from life, there's nothing quite like it.