Regarding stock agencies (Science Source was mentioned earlier), I might suggest that you consider Science Photo Library. SPL is based in England but had agents worldwide. Science Source handles the SPL collection in the US, for example. I've been with SPL for a long time - so long I can't remember how many years. Royalties fluctuate a lot and after several years of increasing turned around and went downward. I think because of the proliferation of other stock agencies, royalty-free options, etc. and having to charge less to stay marketable. Nevertheless, it is a good company to work with and whatever money comes in is welcome.
In the "old days" I'd have to send in 4 x 5 transparencies, which was a pain since a good number would be rejected due to dust or scratches. But today, good digital files are all they need. You can't send the same images to other stock house that you give to SPL, of course, but you can still negotiate a use of the same image to someone who comes to you directly.
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