Thank you to everyone who replied, and to Mike (N1MG) for confirming that he's also able to see the images posted from my webspace. Looking at the responses it now looks as though it's all down to individual browsers and settings. An immediate solution is to use an external image hosting service., despite my preference for using a personal webspace. My ISP does offer a paid secure hosting option and I'll consider that in the longer term.
73, Peter.