A WebmasterWorld thread brings up a topic I've honestly never considered before (which is why I love SEO forums). Here is the situation...
You have a web site with lots of images, let's say an e-commerce site with lots of product images. You decide to replace all the old images for each product with new fancy images.
The question is, what do you do with the old images? Options are:
(1) Leave them on the server.
(2) Delete them from the server.
Now, if you decide to leave them, all it will do is eat up your bandwidth. That is unless they somehow continue to display on the original product landing page and those somehow convert.
If you decide to delete them, then you have a things to consider. Do you just delete them and upload new ones or do you delete the old ones and replace them with the new ones but keep the same file name?
For example, you sell blue widgets on a page. That page has an image of a blue widget at domain.com/images/bluewidget.jpg. Do you replace the old one with a new image on a new file name, i.e. /images/newbluewidget.jpg or just overwrite the old one with /images/bluewidget.jpg.
What would you do?
Update: Google's Pierre Far responded to my question on Google+ with the answer:
You can keep the same filename if you're just updating the image of a product - i.e. the new image and the old image are about the same thing. As Googlebot re-crawls the images (assuming there aren't any robots.txt disallow directives), we'll start showing the new images in the search results. How long it will take to refresh all images on a site depends on a lot of things.
Forum discussion at WebmasterWorld.