What @bwiernik said, but:

I have photographs scanned from books...

In that case, you would probably cite the book as the main work, and then cite the image by page in the book when inserting a specific citation. (Zotero also allows, for example, citing a "Figure" rather than "Page" -- click on "Page" to open a dropdown menu in Word when inserting/editing a citation.)

Of course you could also just add an image from a book as a separate 'artwork' entry, but that would only be if you don't care about the book (for example, a textbook of Art History and you're just adding the Mona Lisa, or some famous photograph, or whatever, based on the info there, not "about" the book at all).

...and photos from online

Again, I'd probably cite those by webpage as the main work. Then specify the image. But you could cite each image specifically if you consider it to be the main work (again, for example, citing the Mona Lisa after finding an image of it on Wikipedia).