This is a great feature in Mail that I've seen, but never realized what it did until I stumbled upon it today -- you can now resize image files right in the composition window.; no more exporting from iPhoto. When you're composing an email with an image, Mail now provides you with a little drop-down menu in the lower right-hand corner of the window named, aptly, Image Size. The default value is Actual Size, but changing the value to Small or Medium will resize the displayed image and the file size (Large didn't make much, if any, difference). If you have multiple images in the message, they are all resized to the chosen size -- you can't change images individually, but it's still a great time saver.
This may have been obvious for some users, but I always assumed it just altered the preview display and not the file itself (like toggling between viewing "icon" and "in place") - so I figured I'd pass the info along.
[robg adds: Even though this is a hint on an obvious feature, some may have overlooked it. The good news is that it's amazingly convenient. The bad news is that Mail isn't as efficient as it could be when resizing images, and I wouldn't recommend using this feature ... read on for my analysis.]
[robg continues: Why not use Mail to resize image attachments? Well, Mail is quite inefficient at resizing images. For my test, I started with a large, somewhat complex image -- a 1920x1200 version of this picture. When I attached it to a new message, Mail stated that the original was 881KB in size. I then resized the image to each of the three sizes in the pop-up, control-clicked the new image, and chose Open With -> Safari to get the size of the new image. Separately, I then used Photoshop Elements (PSE) to resize the original to those new sizes, saving each as 100% quality JPEGs (via File -> Save for Web). Here are the results:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050730200838947