After acquiring images for your website, you'll need to reformat them since they're probably too large and possibly in the wrong file format.
The general rules are:
To get the complete low-down on formatting images for top results in the search engines, check out this great article by Darren Rowse at Problogger.net:
http://www.problogger.net/archives/2005/03/12/formatting-images-for-seo/
TIP: Remember to keep the original image as-is, and file it in your archives – you may want to reuse the image in another format or on another website. Once the image has been reduced you won’t be able to enlarge it later without losing definition.