The problem:
Images and other static files will not be cached unless you tell it to. In IIS7 it is remarkably easy to do this.
Web pages are becoming increasingly complex with more scripts, style sheets, images, and Flash on them. A first-time visit to a page may require several HTTP requests to load all the components. By using Expires headers these components become cacheable, which avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often associated with images, but they can and should be used on all page components including scripts, style sheets, and Flash.
Every time a page is loaded, every image and other static content like JavaScript files and CSS files will be reloaded on every page request. If the content does not change frequently why not cache it and avoid the network traffic?!
The solution:
In IIS7 there are two ways to cache content, using the web.config file to set caching for all static content, and in IIS7 itself setting aching by file extension that gives you that extra level of granularity.
Web.config:
In IIS7, Expires Headers can be enabled in the system.webServer section of the web.config file:
In the above example a cache expiration of 1 day was added. It will be a full day before the content is downloaded from the web server again.
To expire the content on a specific date:
This will expire the content on December 31st 2011 one second before midnight.
Issues/Challenges:
Once the file has been set to be cached it wont be updated on the user's browser for the set cache expiration. So be careful here with content that may change frequently, like during development. Typically in development you don't want to cache at all for testing purposes. You could also suffix files with timestamp or versions to force a reload into the user's browser cache.
IIS7 Expire Web Content
Open up your web app in IIS. Open up the sub-folders until you find the folder or file you want to ad an expiration date to. In IIS6 you used to right-click and select properties, no such luck in IIS7, double click HTTP Response. Once the window loads for the HTTP Response Headers, look to the Actions navigation bar to the right, all the way at the top select SET COMMON HEADERS. The Enable HTTP keep-alive will already be pre-selected. Go ahead and add the appropriate expiration header to the file or folder. Note that if you selected a folder, it will apply that setting to all images inside that folder and all nested content, even subfolders.
So, two approaches, depending on what level or granularity you need.