In my Google Webmasters account, in Optimization section, under HTML improvements I see that Google has indexed more than 100 links complaining that these links has no Title associated with them (Missing title tags).

However, these links all result in thumbnails of images and they are served as MIME type (Internet Media Type) of "image/png", or "image/jpg", etc.

What's wrong here? How should I tell Google that an image can't have a title tag?

Note: An example link can be see here.