Personally I think Veer has the worst watermark - the only purpose it serves is to advertise their site rather than offer any sort of protection to the image.
The standard preview/watermarked image on Veer is about 400px, which is pretty much the same dimensions as a 1 credit file (Xsmall) on iStock - so what's to stop someone downloading it free on veer rather than purchasing it legitimately? It would only take a couple of seconds to crop/clone the corner.