There are plenty of good online image editors, but uploading an image for simple tweaks can be painful, especially if the whole aim is making a picture easier to send by reducing its file size. ezyImageResizer lets you resize local images within your browser.
The Java-based service is pretty simple to use: add or drag on the images you want, specify the percentage reduction and an output folder, and the resizing begins. Output files keep the same name, but with a '-resized' suffix added. The process isn't super-quick, but this is still a handy bookmark if you want to do some quick resizing and don't have a preferred desktop application installed.