This is a complex topic that extends beyond the scope of this thread. "Stealing" in a digital context is a bit odd, since things like images can be replicated. So whereas stealing a fruit deprives the owner of a fruit, utilizing a copy of a digital file deprives the owner of whatever benefit they may have obtained due to owning the exclusivity of that file. This makes sense when the owner is generating a profit from the image -- in which case exclusivity is key to revenue generation.
In the software world, individuals post snippets of code (or even entire programs) throughout the net -- be it on blogs, source code repos, stackoverflow.com, etc -- literally billions or trillions of lines of code -- and it's understood (and even encouraged) that readers are able to copy that code into their own projects, whether it be revenue generating (ie, for an employer) or simply a re-contribution to the vast repository of online code. So, it's perhaps a bit strange that photos posted to a blog or to a forum like this one are governed by a totally different set of rules (both legal and societal apparently?).
There is another angle to this -- which at least one poster touched on with a reference to "sharing plants." Hoarding possessions, whether it be generic material, images, source code, etc, generally precludes proliferation and popularity. If one has a stellar image or a novel cultivar with extraordinary characteristics, "sharing" it with the public is a great way of popularizing it, and perhaps even generating revenue from it.