I understand your request for a simple 'Mail' Button Bar button but, working with what is currently available, the following workaround may suit your needs better (though is a little more complex that my first suggestion).
It uses the 'Open Image' button (displayed on the Button Bar) so if you currently use this button for its intended purpose, you will not be able to implement the following.
Set every image's linkURL to 'mailto:email@address.com'.
Set every image's linkTarget to '_self'.
Set showOpenButton="TRUE" in your gallery's XML file.
Change the icon for the 'Open Image' button using the technique in the Using Custom Icons support section.
Change the toolip text for the 'Open Image' button using the languageList configuration option (so as not to confuse users).