put the following code in web.config file->
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<configuration>
<appsettings>
<add key="imageAddress" value="../HowToOpenPDF/Styles/Site.css" />
</appsettings>
</configuration>
and use the following code where you want to access the image with name->
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ConfigurationManager.AppSettings["imageAddress"].ToString();
or using javaScript on button click:
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<asp:button runat="server" id="btnConfig" text="Read From Web Config" xmlns:asp="#unknown">
OnClientClick="ReadFromWebConfig()"/>
function ReadFromWebConfig() {
var hk = '<%=ConfigurationManager.AppSettings["imageAddress"].ToString() %>'
alert(hk);