istock  are clawing back with a little marketing this month, along with a double royalty day for exclusive photographers they are offering double ($20 instead of $10) for members who refer a new member during September. New referred members are required to purchase pay as you go credits or a subscription.
Despite the offer, if you have a customer wanting to buy your images and can get them to use the referral codes (from your site, profile or via an email) then I still recommend the crestock referral scheme for its 50% payment, or the fotolia scheme for their 10% over 5 year "long tail"
If istockphoto can afford to pay out more during September then why not every month? Why not a percentage of the actual money a new member spends?
More: our istockphoto review