I recently created some very simple templates that are used for announcements and messages in more of a letter/memo form. However, I've noticed that Gmail adds what looks like a light yellow overlay to the header and footer images. No other client does this.

Thoughts? Very crude example is attached (best I can do from my iPhone at the moment). I'm sure it's something simple, but at first glance I can't tell. Will post the code later if it's not obvious.

Yellow overlay screenshot