I've never noticed this before, I think this is new to either Safari v1.1 or Safari v1.2. Anyway, if you've ever wanted to know the size of an image on a web page without downloading it and looking at it in Preview (something I do every now and then), there's a really easy way to do it.
If you open the image in a new window in Safari (which can be done through the contextual menu if it's on an open web page), the title of the window (or tab) now shows the image size in pixels.
[robg adds: I believe this is new in Safari 1.2, and it basically makes this hint somewhat redundant, although the referenced script will also tell you the file size of the displayed image.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040204151315934