And for several years now, Google has had a tool at our fingertips to help us stay legal. So, use it.
More opinions on the subject, and a nice explanation from a lawyer, can be found with a quick - what else? - Google search. Although it's not the freshest piece on the subject, I'm fond of CNET's article on the subject.
I don't like thieves, and I definitely don't want to be one. When managing websites and social media accounts for my clients, I opt for original art first, vendor-supplied art second, and then search for images "labeled for reuse" - and credit the source in most cases when I use them. Need some help managing content that will get you more sales - and no letters from lawyers? Get in touch.