Apple has published a tip
on re-sizing photos in Mail “on the fly” which was picked up by TUAW
(and others).
It points out that if you drag a full-sized photo into a new message window, you can then resize it using the drop-down menu in the bottom righthand corner to Large (1280×800 pixels), Medium (640×400) or Small (320×200) sizes:

Now, obviously, any Mail tip is a good one by definition and any resizing is better than none for your Sent Mail folder, your bandwidth and your recipient, but there is one thing that the tip doesn’t mention.
Mail sucks at resizing images.
A year ago, Rob Griffiths ran the tests and did the sums
when the same tip was posted on macOSXHints. He discovered that Photoshop is between 61% and 73% more efficient at the same tasks. Even iPhoto itself does a better job.
Not everyone owns or likes sledge hammer apps like Photoshop, but if you do this a lot, you could consider utilities like Downsize or Scale to Mail, which will resize your images efficiently without the need to fire up the monster.
If you are reading this but for some perverse reason don’t use Mail, resized photos may still be within your grasp. If you use Claris Emailer, GyazMail, Mailsmith, Outlook Express, PowerMail or QuickMail Pro, Simon Jacquier’s iPhoto Mailer Patcher lets you hack iPhoto to set your email client as the recipient of resized photos.
Tags: Apple Mail, Apple Mail Tips, GyazMail, images, iphoto, mail.app, Mailsmith, photos, powermail, resizing
What about Firefox? Any similar plugins you know about for that?
One of the problems with the apps out of the Mozilla stable is a lack of “applescriptability” which many of hacks and plugins depend upon. That’s the case in this instance too.
I can’t do Apple Script anyway, it’s too difficult, isn’t it? I know there are thousands of extensions for Thunderbird and its stablemate, Firefox. I guess I’ll just have to look for one of those.
You may also want to consider ImageWell. It’s a small app for (you guessed it) resizing photos. I don’t know if it is better than what’s built into Mail, but it’s certainly fast and easy. Check it out here:
http://www.xtralean.com/IW.html
Here are the two I decided to get in the near future.
EasyCrop for one-offs, Sizerox for batches:-
EasyCrop at http://yellowmug.com/
Sizerox at http://www.sizerox.com/
I recommend this sweet tool Sizerox (SizeRox). It’s user friendly and has a lot to offer. The presets are very helpful.