I have always just assumed that you didn't have publication rights to good images of EVERY coin in existence, so you simply used a similar type image in their place. Not a problem for me, as I have simply replaced the stock photos with actual photos of my coins. I'm lucky in that I have a full blown coin photography setup and can easily produce my own images.
I know exactly how hard this is, because I have collected images for my coin club's web site. It's a daunting task to find out who actually owns the publication rights to an image, or if the image is considered public domain. Thankfully my needs are for educational purposes only, so the image owners have been great about granting permission.