caching images on browser

I开发者_如何学编程'm running page performance tools in firebug and I keep getting the suggestion to cache images? I thought browsers cached images by default. How do you cache images?

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Here's how I cache in IIS7

<system.webServer>
    <staticContent>
        <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="90.00:00:00" />
        <remove fileExtension=".js" />
        <mimeMap fileExtension=".js" mimeType="text/javascript" />
    </staticContent>
    <caching>
        <profiles>
            <add extension=".png" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" />
            <add extension=".jpeg" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" />
            <add extension=".jpg" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" />
            <add extension=".css" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" />
            <add extension=".js" policy="CacheUntilChange" kernelCachePolicy="CacheUntilChange" duration="00:00:30" />
        </profiles>
    </caching>
</system.webServer>

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it's just suggestion; browser will cache images by default,don't worry!