However, if I post the URL for the picture, although it initially appears at the same size, it then reduces to a smaller size.
Heliport, I assume you are using Internet Explorer. If you click Tools -> Internet Options, click the Advanced tab and scroll down to Multimedia, the "enable automatic image resizing" option is checked - correct? IE is rescaling the image to fit in your window, and, for some reason, this doesn't trigger on PPRuNe (probably because of the way Tables are used to display individual posts).
You'll need to rescale the picture, or ask the owner to do it - 640x480 is usually a good size.