If you look into any app bundle folder names are capitalized

The Resources folder is used for assets like image, everything except .py files.

This is handy in combination with an ExtensionBundle object

from mojo.extensions import ExtensionBundle

myBundle = ExtensionBundle("MyBundleName")

# assuming in the Resource folder there is a file called "myToolbarIcon.png"
# when retrieving files from a bundle an extension is not required
# this will return a NSImage object
toolbarIcon = myBundle.get("myToolbarIcon")

see:
* http://doc.robofont.com/extensions/building-extensions/
* http://doc.robofont.com/api/mojo/mojo-extensions/

the ExtensionBundle us written similarly as NSImage.imageNamed_

from AppKit import *

print NSImage.imageNamed_("ExtensionIcon")