I use another
Turns out you should probably keep your icons in an "
.iconset". And have all the recommended sizes available for the notification center to pick up (e.g. might be 32 x 32, might be 16 x 16).
And to create this, you can use the "Icon Composer" app to convert your .icns file into a full blown icon set. The .icns file does end up in your built app package / bundle in the end, though.
Details can be found in Apple's "High Resolution Guidelines for OSX" document, under the "Create a set of icons that include high-resolution versions" section.