Caching of .png,js and css

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 :)