secure message in IE for ASP.Net website

can anybody please tell me, why this no-secure message comes on my site's homepage in Internet Explorer Following is message:

"T开发者_StackOverflow中文版he web page contains content that can not be delivered using secure HTTPS connection" message box comes with yes/no option.

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Your webpage is using https but some items (picture for example) on the page are linked using http (not https). This is why IE warns users that although most of the page is secured not all content on it is. If you want to get rid of the message you should serve all the items on the page via https.