permission is still needed from the people who created the artwork in question - to me whether or not they are selling it is beside the point (another issue altogether). what is so damn hard about asking for permission? i have 2 online galleries i post my work at and a few other websites that have an authority to do so. all of them has given proper credit and those who have cropped my render have the power to do so since they paid for it.  it really boils down to the principle of it all - give us the courtesy to have a choice whether or not we wanted other websites to house our work or not. to me it is one way of keeping track of them.