After a lot of communication on this forum about refunds, I just want to explain what is happening in background of it.  I don’t  want to tell that IS is doing that on purpose, but IS  does nothing to stop it, because they don’t  lose anything.
IS made a banner which contributors import in their site. When someone click on that banner it transfers on IS site with form for registry. After registration new customer have 10 credits as a gift.  You can open 20 different email addresses or more in a day and win 200 credits or more for legal photo buying.  A lot of people are buying photos on this way.  After 2-3 weeks when IS figure out what happened, when customer didn’t buy any credit package, management decide to take out money made of these sales from contributors as refund. Istock don’t lose anything, but contributors gave their files for free.  A lot of contributors have more than 50 refunds in a month.
What is consequence with these 10 credits?
What are you thinking about this?
A lot of people have refunds always same amount and it is from 10 credits gift.
Let’s talk about this.

But what I don't understand is what "percentage" they were paying in the first place, since they've said for years that they only pay commission on the actual cost of the credit. Surely our pictures would have been given away and nothing entered on our sales list in the first place, if this is right?

No. While discounts (like the usual "our site is down, take 10% on your next purchase") are part of the royalty calculation, the free discounts were always valued at a fixed rate in the system, so the contributor gets a royalty on them.