It mostly like the image file on the file has been removed or something is wrong with URL. So could you please try to download the image from browser, eg: IE browser, to see whether the URL is correct to download the image file? Thanks.
By the way for the URL, please check both the URL written in image tab and in entitlement. by default, the URL in entitlement is the same as the one in image tab. if you ever changed the URL in entitlement, the URL in downloading list will point to the URL in entitlement.