Google doesn't guarantee that our system will index all the URLs in a Sitemap, for many reasons. It's rare for every URL on a site—particularly a large site—to be indexed, and in general, fluctuations in the percentage of pages indexed is not something to worry about. (Focusing on improving clickthroughs or conversions on your top 20 URLs can often have far more impact on your site's ranking than striving to get 100% of your pages indexed.)
In addition, we don't index images directly (instead, we index the page that contains the image). As a result, direct image URLs in your Sitemap won't be indexed.