<system.webServer>

<rule name="404 Images" stopProcessing="true">
    <match url=".*" ignoreCase="true" />
    <conditions>
        <add input="{URL}" pattern="^.+\.(jpg|jpeg|png|gif|ico|bmp|tiff)$" ignoreCase="true" negate="false" />
        <add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="true" negate="true" />
        <add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="true" negate="true" />
    </conditions>
    <action type="Rewrite" url="/Files/image404.html" />
</rule>

..............................

  <httpErrors errorMode="Custom" existingResponse="Replace">
            <remove statusCode="404" subStatusCode="-1" />
            <error statusCode="404" responseMode="ExecuteURL" path="/PageNotFound.aspx" />
   </httpErrors>