The Image Sitemap plugin will generate an XML Sitemap for Images that exist in your WordPress blog.
[v2.1.3] Minor fix related to permalinks that could be empty. Thanks Andrea Gianfreda for the tip.
[v2.1.2] Compatible with WordPress 3.5
[v2.1] Data values (including URLs) now use entity escape codes for certain characters.
[v2.0] The plugin has been significantly improved and now includes all images including photo galleries, direct embeds and media attachments.