Lo and behold, Tim is right! It was a little obscure, but here's how it works:
[H&kon, take notes :-]
right-click any image (or, since this is a Mac, control-click)
Select Images/Filter and then in the dialog box that follows you can say
Filter all images of this size, in pixels
all images from a given server
all images with a common path
just this specific URL
Then, go to the main menu bar and select the toggles for "Filter
Images by URL" and "Filter Images by size".
Lo, and behold! It just works!
iCab's cookie handling is almost as powerful, too