All of the images on this site are originals and are copyrighted by BGA Graphics, with all rights reserved. I hope you can understand the importance of giving credit to the artist.
Most importantly, you're required to have the permission of the artist in order to use any image from any source for any purpose.
You shouldn't download free images from obscure sites or individuals because it is difficult to know where the original came from. If you were to use an image that was lifted from a copyrighted collection, you would likely suffer the penalty. You should deal only with established companies under a clearly stated use and copyright policy.
Copyright is a protection that covers published and unpublished literary, scientific and artistic works, whatever the form of expression, provided such works are fixed in a tangible or material form. This means that if you can see it, hear it and/or touch it - it may be protected. If it is HTML coding or a computer graphic that can be set on paper, recorded on tape or saved to a hard drive, it may be protected. Copyright laws grant the creator the exclusive right to reproduce, prepare derivative works, distribute, perform and display the work publicly. Exclusive means only the creator of such work, not anybody who has access to it and decides to grab it.
If you'd like more information, please visit the U.S. COPYRIGHT OFFICE web site.