It's just the angle of the primary image. There is no right or wrong way, you can have the curve going up, down, side to side or twizzel it like and umbrella. The choice is yours, it all depends on where your sweets spots are. Each product has an image of the dildo's in the palm of a hand and each one has the head pointing upwards
I agree with K&c30's. I own the dildos in the last 2 links and usualy use them with a strap-on harness. I place them so that they curve upwards, like most real penises do. If you are holding them in your hand or if you ''stick'' them to a surface, you can rotate them so they are curved the ''right'' way for you.
I hadn't noticed it before, but the fact they they are curved downwards judging by the shape of the head makes no practical difference. =)