The reason some images appeal to us more than others, is that they more accurately represent the idea we would like to express when using an image. Very often they are of a quality we might not be able to achieve ourselves or take more time than we have to produce them.
Photographers and artist spend time, lend experience and vision to the work they create. It may not seem like stealing when you use an image you like from the internet because there are really few barriers or specific warnings, but these work products have value and must be paid for. The great thing is that there is a broad variety available, and many are not very expensive.
Stock Image houses are getting much better at tracking down the products they license and are becoming stricter about the consequences. Check out this article about Getty images pursuing copyright infringers in court.