I'll start with an obligatory reminder that I am not an attorney.

However, I have to discuss what we're doing regularly with an attorney. His advice to is basically the following:

1) Courts in different countries (and also laws in different countries) deal differently with copyright issues, so it's best to learn how these issues are handled by the courts and laws in your jurisdiction. He has told us, as an example, that an action by a court in the U.S. against us here in Germany would be ineffective if it was based upon copyright law or case law in the U.S. that is inconsistent with law or case law here.

2) In general, he said, copyright violation assertions can succeed for individual images only if they are duplicates of copyrighted material.

3) However, he said (as attorneys always do), if one produces a collection of images that almost precisely duplicates the style of someone else's copyrighted images, it is possible that the original copyright holder can successfully argue in court that your work is not original but derivative and therefore a copyright violation.

Now, don't take that as legal advice, because it isn't legal advice. It could however serve as a good starting point for your own pondering upon how you want to proceed.

If it looks to you like an assertion of copyright violation might succeed in your case, it would be wise to consult a local attorney with copyright knowledge to advise you on how these things are handled in your legal jurisdiction. I say "attorney with copyright knowledge" because there are attorneys with no experience whatsoever in copyright matters who will be glad to take your money in return for ill-based advice.