You can use the file unix command in the Terminal to quickly find the dimensions of (most) images.
karma:~/Sites/somepics me$ file *
yel-pale-grad.gif: GIF image data, version 89a, 20 x 125
m46.jpg: JPEG image data, JFIF standard 1.02, resolution (DPI), 200 x 200
troll.png: PNG image data, 206 x 188, 8-bit colormap, non-interlaced
This is a great help to web developers like me that like to put the image size in img tag. (BTW, I've found some jpeg images return just an aspect ratio of 1x1, but most of the time this works.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031124140353993