When optimizing your landing pages, or any website for that matter, page load speed is a big factor. It is said that 4 seconds is the maximum amount of time uses/buyers would want to wait before they get annoyed.
(http://www.akamai.com/html/about/press/releases/2006/press_110606.html) You could see a drastic change in conversions by simply making your site load faster. Internet users are accustomed to buying things form large sites, like amazon and ebay, and thats the sort of load time they have learned to expect when accessing other sites. These companies usually have a whole team dedicated to optimizing load time, and usually drill down to the milliseconds.
Here is a sample image I optimized.
Even the smallest change helps here. Imagine if Google saved just a byte from optimizing their default Google image. Millions of people hit Google everyday, the bandwidth they save would be massive.
If you have any sites, start form optimizing your logo (as that is the one that gets loaded most frequently). You could then upload all of your other graphics to smash.it and just replaced the ones with the optimized version you download.
This is a great tool to forward to your designers/developers.