This is what I'm seeing when testing it and found that all my tracking images (tracking pixel) have hits as below.

Remote address: 66.249.x.x [any google ip]

Referer: [not set]

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.0.7) Gecko/2009021910 Firefox/3.0.7 (via ggpht.com)