Most of the design elements in my microstock image collages I create from scratch or generate in software programs. I feel uneasy to use internet images resources since their copyright status is very often unclear. Yet from what I understand it should be quite save to use illustrations from pre-1923 books and publications as part of image collages or as sources for tracing. I will appreciate user feedback on this issue.

In my opinion, if you didn't make it or own rights to use it, please don't mess with it. Even though there are legal loopholes, there are so many people stealing art (music and images), that we shouldn't perpetuate a bad thing. Take some inspiration from the old books, but do your own thing.