Is it possible to withdraw a portion of your istockphoto earnings and get them paid into a 3rd party's account? I am buying some equipment from a US seller and the easiest way for me to do this (as UK based) would be to deposit a portion of my istock funds directly into his account.
No. Since you can't deposit directly into your own bank account, it's safe to assume that you can't deposit it into theirs. However, if you list their paypal account as the account you want Istock to send the money to Istock pays the fees that wouldn't be an issue.
What would be an issue, at least in my book, would be the lack of receipt proving that you paid for something from the merchant.