Image not rendered in vf page with content type is excel

I have one VF page which has content type excel means its for Export to excel, now i want to show one external image in that excel but not rendered in excel image URL is correct cause while i change content type of VF page as HTML i can show image , how can i resolve this issue 

and in this VF page i used workbook , is there any issue regarding workbook? Please reply me ASAP.