This will probably be a no-brainer for most of us but the easiest way to improve the speed of a website is to limit the number of images on the page.
There are still sites out there where it's obvious a graphic designer has let rip with Photoshop before slicing up their creation to make a web page. In the (recent) past I've worked on sites where the customer has been desperate to change the way the menu works as adding a new "button" meant they had to pay the original graphic designer to create a new one. They always seem to think this is normal !
Whereas the original homepage loaded 48 images the new version only has 18. This might still seem a lot, but the more you remove the less "designed" the site might feel and so a compromise needs to be found. The trick is to simply remove the unnecessary images. That normally means the images that are nothing but type and can easily be replaced with plain text.
Other images to go from the website were the numerous uses of spacer.gif. CSS can now do the job of tables and spacer images and save some page weight in the process.
If you find yourself using a lot of icon-like images on your page (as codestore does) then it's probably worth using CSS Sprites. In explaining the need to reduce the number of images to my client I talked about how the browser can only request X number of images in one go and the server can only respond with X-n images at once (more on that later in the week). They don't all get delivered at once and so a queue forms and page load time increases drastically.
Another way you can improve page load time is to make sure all the images you do use have their dimensions set properly. If the browser doesn't know how big an image is it has to load it to find out. It needs to know this so it can render the page properly. Some browsers wait to see how big all images are before displaying the page whereas IE will display the page and then it will appear to jump all over the place as the images load and dimensions are recalculated. Although the actual load time doesn't change it's the perception of the user that it's taking longer to load and settle.
To see if you have images with no dimensions install the Web Developer Toolbar for Firefox. With your page open disable stylesheets (makes the next step easier) by pressing Ctrl+Shift+S. Now choose Images > Outline Images > Images Without Dimensions from the toolbar. Those with the red border need fixing!