Based on my many years experience as an art director working with stock
photography and photographer's photos, I can tell you what I know from the
advertising industry. I'm sure it's similiar if not right on for your
If you paid for these slides once for educational purposes than you are
most likely free to dupe the slides as many times as you want. I also believe
that this extends to digitizing them. We did this all the time at my various
ad agencies with out any penalties or fines.
With rights/copyright you really only get into trouble when you try to use
a piece of artwork beyond the intial agreement. So, if you were going to slap
artwork from these slides on a coffee mug and then sell them, you would be
subject to usage/copyright infringement.
As for as cheapest method. YOur scanning in of the images is probably the
cheapest, but far from the best quality. A pre-press service in your area
with a professional slide scanner would get you the best scans but at a much