Internet is full of images, however, it is not that easy to find totally free images. In case you don't create your own pictures or buy them, you can be never sure. Stealing images is very common, but it is not nice to the owners of the images and especially it is illegal. Have a look at our stock image gallery that currently contains over 15,000 professionally designed pictures and photos. You can download many of them freely under Royalty Free licence for a non-commercial use.
Compare these two book covers. Which one do you like more? For many people, the book cover just matters. Keep in mind importance of the design! It make a difference between selling and not selling your products, regardless what inside is.