I teach the History of Modern Art and I have a lot of images that I’ve bought in the past on 35mm slides which I want to digitise. Is this OK?

It would be highly unusual for the sale of the slides to be made on any other basis than that the images be used only in their slide format. Any 'alteration' of the images including digitisation would therefore not be allowed under the original agreement.

You could contact the rights holders for permission to digitise the slides for educational use. Information about the rights holders will probably be located with the slides.