Sorry it took so long to reply to this.  With the Community Edition, you can specify a location for all providers.  This is available in the Settings for the metaPost module.  Remember that you have to log in using a host account to see this module. 

With the Professional Edition, you can set the image path on a per provider basis.  This is done by clicking the Administration link that you'll find in the same location as the Settings link (again, you'll need to be logged in with a host level account). 

HTH,

Don