Great article. One of my favorite resources, Project Gutenberg, offers some ideas on what is "free". I refer to Gutenberg for a lot of my research. Mostly public domain, mostly available for copying, with some caveats. You can use text and also capture images in screen shots. Follow the guidance of Gutenberg in doing this. Rules vary by country. I captured the image here from Lewis Carroll's Alice in Wonderland (copyright 1916). In most cases, I like to attribute material because I think this is courteous and also because readers often like to know sources. Usually, if readers see you've taken care in preparing your material they are likely to place greater credence in what you're offering.
Courtesy is something most of us completely dismiss! We should be grateful that the perfect image for our need is openly available. If we can thank people for holding a door open for us, we can surely thank them for providing imagery for us regardless of the legal position!