I have a site with several thousand images. All those images are included in the sitemap submitted to Google Webmaster Tools. The number of 'submitted' images is OK, but the number of 'indexed' is significantly lower and it is going down!
I'd understand if not all of my images got indexed (however that is also not clear and very frustrating for me) but I can not understand how the indexing can go in the negative direction?!
All the images stay in their place and the pages containing them remain unchanged. At least, that is the intention.