If you take anything thats not yours its stealing. That means using a photograph that you do not have permission to use is stealing. Its that simple.
That works for tangible items quite nicely. Putting media on the internet is a means of distribution. From your computer to mine through a common server. It's not a one off delivery... The file doesn't disappear off your hard drive and appear on the host's server for everyone to view behind glass. When you upload anything it's automatically a copy which is then copied and stored by every browser that views it.
Within moments of an image hitting this forum there are thousands of copies on people's hard drives across the world.