Cheers Scott. I will use the re-sizer you recommend. It is a shame though, as I would much prefer to "attach as a thumbnail, and people can click it if they want it to be bigger", so that the quality of the picture is preserved.
It is also frustrating that it has taken 6 days and 4 pages before anyone accepts that I am describing an issue/failing with the forum system, rather than me just being a luddite dimbo (still a possibility, I grant).
I hope that the admin will continue to address this and work out what the problem is.
Some additional FYI - I am using the "Full" site and I have also tried using the Firefox browser with no better results.