Is there a DOCUMENTED way to use a static resource as an image in a formula field?  I'm trying to create an 'action link' that looks like a command button and need to find a place to store the background image for the link in a way that is environment agnostic

I'd hoped this would work, but $Resource doesn't seem to be recognized in formula fields (works fine in visualforce formulas).  

HYPERLINK('/apex/createAgreement?id=' +  Id ,  IMAGE( $Resource.Create_Agreement_Button , 'Create Agreement' )  , '_top')

It appears that this will work, but I'd rather not depend on undocumented functionality to accomplish this.  Particulary given I'd like to add this to a managed package at sometime and in theory there could be an identically named resource that alread exists in the org.

HYPERLINK('/apex/createAgreement?id=' +  Id ,  IMAGE( '/resource/Create_Agreement_Button' , 'Create Agreement' )  , '_top')

Documents are also a reasonable approach, but I don't know how to use them across multiple environments where I won't no the document id.