Yes this is copyright infringement.  Furthermore Amazon will remove your title from sale if they ever find out that you do not have permission to sell it and if the the artist contacts Amazon in regards to the infringement Amazon will prevent you from ever selling your title again or any other content.  

Depending on which region of the world you could also face royalty fees associated with using this image without the proper rights.  In America this is known as "damages".  

Now the likelihood of the artist finding out may be minimal but as an artist in the field would you like it if someone ripped off your work?  Do the right thing and either contact the seller or spend the few dollars for it since you already found it on iStockphoto.