Is Google causing problems? I have been getting the following notice from Google's IP address:


  Unable to generate the derived image located at public://styles/large/public/field/image/imagefield_b9xgKB.jpg


Why does it happens? It started a week ago, and now it happens few times in a day. Sometimes the filename varies.
This is an error saying you have a link to an non-existing resource. 

You should use Webmaster Tools to dig deeper, if I remember correctly, it should provide the URL for the page with the bad link.