I'm in the process of getting a cold.... achoo... wasn't going to post today but saw this in my inbox and got me to thinking.
You can now put pictures on a stamp - they are a bit expensive but if you mail a lot of things for your business (advertising / etc.), it could make the difference of someone opening it and not. How much of a difference the only way you would find out would be to test it.
Just make sure that the only change on the envelope is the stamp.
Have a good one.