Expiration and caching response headers
Browsers reload content based on their last accessed expiration and caching headers sent from the web server. So if images expired in the past, browser will have to reload them regardless of whether they've just been loaded.
As you're using IIS I suggest you read this question that links to MS resource with description of configuration elements to control this behaviour.
When you configure your server to not expire static content you can still convince browser to reload some static content by adding a random value along with your request i.e.
http://somewhere.com/images/always-realod.jpg?a9s87h7sdf6