Thanks everyone. I've already applied as much compression as possible without loss of quality and the files were still 4-5Mb each. I experimented a little and found that physically reducing the image size (they're VERY big!) made a lot of difference, and brought the file size down to 25% of what it was.
Thanks especially for that comparison on file types, Jhieminga, that was very useful.
Thanks again, Bluey.