Rails 3.2 - how to write an image path in a config/initializer script?

In my config/initializers/payday.rb , I need to write an image path to
asset image …

Currently it’s written like that :
Payday::Config.default.invoice_logo =

how can I set the path, as asset_path cannot be used yet during
initialization ?
if not possible I guess I should have a copy of the image in the public
folder ?

thanks for feedback