As of February 20th, Google Affiliate Network will no longer support the use of external image URLs for links. After February 20th, all advertisers will need to upload creatives through the interface before they can associate them with links.
Here are instructions for how you can upload a creative: http://support.google.com/affiliatenetwork/advertiser/answer/1272987
After February 20th, any images currently referenced by external URLs will be hosted by Google Affiliate Network.
News and Information for Google Affiliate Network Advertisers
