There is a compelling argument that it is . Celebrating(if that is what was happening ) or marking the onset of shorter days is less likely than the converse but most importantly the sun doesn't set over the heel stone (the obvious foresight and in the middle of the avenue ) as seen from the centre of the monument , it's off by about three sun diameters to the west . From the Heel stone with the axis of the avenue behind does provide a more accurate alignment for the setting midwinter sun , an erect altar stone would have been a perfect line of sight but that is conjecture .The sun sets on the horizon at the eastern side of stone 56 of the great trilithon . I don't know of any pics but there must be some .The problem with pics is that you don't necessarily have the info of where the pic was taken from i.e. you can show the sun setting or rising over any stone you like but it is wher it is being observed from that is important .