Springer has just released a collection of over 1.6 million scientific and medical images. SpringerImages is based on SpringerLink and images.MD and, as well as the images, provides the source captions, extracts and links to full text.
SpringerImages requires a subscription but there is a growing collection of open access images, free to everyone. You can limit your search to Only accessible images (free + subscribed) and this will provide you with image source details, including a link to the article in which the image appears. Registering gives you access to personalisation features like creating you own image sets, adding keywords to images and saving searches.
Deirdre