Upload your image to somewhere it can be accessed, eg your website, an image hosting service, etc.
In QuoteWerks, go to Tools --> My Preferences --> E-Mail --> Defaults and click on the green button to edit your signature.

 Then, in the edit window, click the 
(H) all the way at the end to see the HTML for your signature.  To have a hosted image for your logo you will need to have an image source entered similar to the following: