Check below links may be it help:
Image Caching in ASP.NET
Static Content Caching ASP.NET
Also use i Web.Config
<configuration>
<system.webserver>
<staticcontent>
<clientcache cachecontrolmode="UseMaxAge" cachecontrolmaxage="365.00:00:00" />
</staticcontent>
</system.webserver>
</configuration>
The above code tells the browsers to automatically cache all static resources for 365 days.
Hope It Helps :)