having knockout graphics on your site is great... but if they take forever to load, you're going to lose visitors. They need to be optimized to get file sizes as small as possible for several reasons:
the smaller the file, the faster the image will load in the browser
smaller files take up less room on the server; if your service provider charges you by the overall size of your site, your monthly charges will be less.
Typically, there is a tradeoff in smaller file sizes... image quality usually suffers. But if it's handled properly it's possible to reduce file sizes by 75% or more with virtually no tradeoff in image quality. The two pictures below show an example of the same image optimized two different ways...
the optimized GIF on the left is 45 kilobytes
the optimized JPEG on the right is 1/4 the size, and is only 13 kilobytes.. a substantially smaller file size.
Although they look identical, the JPEG will load almost 4 times faster than the GIF. If you have a page with 10 or 20 graphics, the penalty for not optimizing correctly adds up to a good looking page that moves like a slug.
Optimizing is not just about file size. Graphics also should...
look good at various color depths on various monitors (i.e. in 256 colors, thousands or millions of colors)
have a density (gamma) that looks good on either a Windows machine or a Mac (for example, the same exact graphic will look much, much darker on a Windows machine)
look good on a small (14") monitor or a large (21") monitor.