file name is suspicious by itself but there is also this:

http://www.istockphoto.com/stock-photo-14024734-hyena-showing-tongue.php

Could this transform counted as substatial? If so its not against license (assuming he payed 2$ for the original image).

http://www.istockphoto.com/help/licenses they have license restriction no matter how much you pay. Thus, AFAICT, it can't be CC0

vk, its not the original image. It is something completely new that is created using the original. And in the license: (from http://www.istockphoto.com/license.php)

(ii) if the original Content has been fundamentally modified or transformed sufficiently that it constitutes an original work entitling the author or artist to copyright protection under applicable law, and where the primary value of such transformed or derivative work is not recognizable as the Content nor is the Content capable of being downloaded, extracted or accessed by a third party as a stand-alone file (satisfaction of these conditions will constitute the work as a “Permitted Derivative Work” for the purposes of this Agreement). For example, you cannot superficially modify the Content, print it on a t-shirt, mug, poster, template or other item, and sell it to others for consumption, reproduction or re-sale. These uses will not be permitted as or constitute Permitted Derivative Works. If there is any doubt that a work is a Permitted Derivative Work, you should either obtain an Extended License or contact iStockphoto’s Client Relations for guidance. Any use of the Content that is not a Permitted Use shall constitute infringement of copyright.

Time is precious especially when you have to spend it on copyright infringement trials instead of your own business (because you used an asset uploaded by someone whose time was more precious than yours)

joking aside:

Time you spent for choosing a proper file name shows you care about your creation.

Time you spent to put a simple sentence about your inspiration/source may avoid possible copyright problems for the users.

Spending those little amounts of time shows you care about the community. Time is precious even little amounts of it.

Sorry I didn't want to offend you, I wasn't asking you to remove it. I guess my approach in the first comment was wrong.