MicrostockGroup

Not for introducing contributors: there are plenty of these already. That has not been working since I joined four years ago.
There is a $10 fee for introducing a new buyer, not via your Moo card, if exclusive.

istock are also operating a referral program on linkconnector (Presumably because it's the same network getty use), details are at http://www.linkconnector.com/pdf/iStock_Promotional_Guide.pdf
(buyers only or course)

whats interesting is 7% of credit purchases for existing customers (and the coupon codes) presumably existing buyers who intend to keep buying just buy credits at istock; so is this partly trying to claw back the price conscious buyers who have already left via an affiliate marketing campaign?

I think the referral link system was broken with the change in the URL structure from the new search.

I used to get a referral commission once every month or two. I can't remember the last time I've had one. Not sure if this is a coincidence.

And I would guess that any site problem that pays out more money is probably at the absolute bottom of the priority list.

Right on both counts.
You don't get a referral fee on the Moo cards because the user gets ten free credits: if they can work out how to get them!
(Easiest way: www.istockphoto.com/friend then type in the code on the card. I have no idea how anyone was supposed to guess that, and assume it was deliberate like the gag about the plans in H2G2.
http://www.planetclaire.org/quotes/hitchhikers