UPDATE from moooooooo try ISTOCKUP25 for 25% off (and maybe other number combinations are possible)
From their email to me.
"For the next 24 hours, get 15% off any iStock credit pack using promo code ISTOCKUP15 on checkout"
I don't often see retailmenot.com has a some perpetual new customer '10%' off and a '5%' of for existing customer codes but you need to be buying 100+ credits for that.
This seems not to be restricted by the amount you purchase.