I've just noticed that this thumbnail:

http://81.5.150.113/wiki/images/thumb/180px-Diagram2copy.png

and original:

http://81.5.150.113/wiki/images/Diagram2copy.png

The thumbnail has only about 6% the pixels of the original, but is still 88% as large. It'd be faster to just have the original.

Mediawiki seems to have a problem with PNG thumbnails - it generates 24bpp ones even the original image was indexed.

Jim