I believe there is indeed talk of Google factoring in nofollow links from reputable sources (Wikipedia for example) however I'm almost certain that blog comments won't be included within this.

We've just installed the server side script 'XML Sitemaps' on our eCommerce site.

The script gives us the option of (easily) creating an image sitemap but I'm debating whether there is any reason for us to do so. We sell printer cartridges and so all the images will be pretty dry (brand name printer cartridge in front of a box being a favourite). I can't see any potential customers to search for an image as a route in to the site and Google appears to be picking up our images on it's own accord so wonder if we'll just be crawling the site and submitting this information for no real reason.

From a quality perspective would Google give us any kind of kudos for providing an Image Sitemap? Would it potentially increase their crawl frequency or, indeed, reduce the load on our servers as they wouldn't have to crawl for all the images themselves?
I can't stress how little of a hardship it will be to create one of these automatically daily but am wondering if, like Meta Keywords, there is any benefit to doing so?