Latest version: 2.0.1
for Firefox 3.0 - 3.7a2pre,
Thunderbird 3.0 - 3.1b1,
SeaMonkey 2.0 - 2.0.*
Shrunked
Image Resizer for Firefox, Thunderbird and SeaMonkey
Shrunked is an extension that resizes your photos before you upload them to a website or email them. It greatly reduces the time spent waiting for photos to upload.


For Firefox users
It shrinks an image to the appropriate size and saves it in a temporary folder. That file then gets uploaded to the website. You'll barely notice.
You can get Shrunked from addons.mozilla.org. It's for Firefox 3+.
For Thunderbird and SeaMonkey users
The next release of Shrunked will also be compatible with Thunderbird 3 and SeaMonkey 2, so that you can also shrink your photos when you email them. It should be out soon.
(There is an existing version of Shrunked for Thunderbird 3, here. I've now merged the two extensions for easier maintainability.)