Optimising image file sizes is a great way to ensure your website loads quickly.
Images must first be prepared before they are uploaded for use in your website.
Step 1
The first part of that process involves creating an image at the correct size and saving it for use on the web.
See tutorial Using Pixlr.com to create images for your website.
Step 2
The second part of optimising an image is to ensure the file size is small. This has nothing to do with the pixel size but more to do with compression. The human eye won't be able to see any difference between a compressed image and one that isn't. The most important difference you will notice are the page load speed times.
Images that a re-sized correctly and compressed load quickly!