I've built a small gmail notifier app myself. There should be a piece of code in the python script which goes:
server = indicate.indicate_server_ref_default()
The .desktop file specified above is the one that determines the icon to be used in the messaging menu. The corresponding "Icon" key in my .desktop file looks like this:
The description for the "Icon" key, as given in the Desktop Entry Specification, is:
Icon to display in file manager, menus, etc. If the name is an absolute path, the given file will be used. If the name is not an absolute path, the algorithm described in the Icon Theme Specification will be used to locate the icon.
In my case, the second part of the above description applies (i.e., the algorithm described in the Icon Theme Specification is used to locate the icon corresponding to "mail-unread"). You can instead give the absolute path to your icon, as in:
or you can try the following (it will work only if your icon theme has the icon, which is rare. That is why I'm using the generic icon "mail-unread".):
I'm not on my machine right now, but when I do get on it I'll tell you where exactly you can find your desired icon and what its name is. I also have links to (the very few) good resources which describe how to integrate apps with the messaging menu, but again I need to be on my machine for that.
If you're interested in the entire code for my app, I've got it on GitHub.