Having fixed the performance problems I've been having (more RAM in the server did the trick), and having worked around the file permission problems (I have to create folders in /assets using ftp, and create the _resampled folder too & chmod them both 777 - this is a server configuration issue that has effected other CMSs too).
I am now stuck with a single final problem that stands between me and a completed silverstripe site. Well, it's hardly a problem really, but I would like to understand what's going on.
When I upload images in the CMS and place them into the text in tinymce, they are always resampled. Even when the size of the image is unchanged.
For example, here's the original image:
And here's the 'resampled' image:
Why does this occur? Is it by design? These images happen to look ok, but I believe that's simply because the original one is a bit over-compressed.