Well, after many more tests I have discovered that TIFF images are displayed Ok. I also discovered that if I rename image.jpg to image.jpeg it too displays well. Only images with extension jpg fail to display. On my MACs, all Jpeg files are named with an extension of jpg, not jpeg. Roundcube fails to recognize extension JPG as a JPEG file even though they are the same. Is there a fix for this?

"During times of universal deceit, telling the truth becomes a revolutionary act" - George Orwell