How much should I charge for an iPhone app photo?

I won't disclose details, but I'm taking a photo that will be used as the primary image on an iPhone application with over 2 million users by a for-profit company. I'm wondering would some solid prices would be? I don't want to charge too much, but these are done professionally, so I don't want to charge too little. I've done some research but can't find any solid answers.

Tell them you want royalties of 0.1cent per copy sold.

2 million users? That will net you $2, 000

$5, 000