Bing search blog offers some tips to ensure the images on a site are optimized for search engine:

• Name image files appropriately – For improved relevance, make sure that the file name describes the image appropriately.
• Alternative image text (alt text) matters – For increased optimization, make sure photos are properly described with alternative text tags, and ensure that test within any images is also
• Watch frame breaking – Sites that attempt to break frames make it more difficult for the image to display correctly within search.  Make sure you’re testing your site against the search engines.