How can i ensure my client's gravatar on my portal is at most a rating of PG

Currently, my default gravatar开发者_运维百科 src does not specify any rating level for display. As such, any user who uses PG and above would not be able to view their gravatar image on the portal - i suppose the default is G. How can i modify the url such that i could ensure client's gravatar would be able to display with at the most a PG rating?

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Check out Image Requests:

Gravatar allows users to self-rate their images so that they can indicate if an image is appropriate for a certain audience. By default, only 'G' rated images are displayed unless you indicate that you would like to see higher ratings. Using the r= or rating= parameters, you may specify one of the following ratings to request images up to and including that rating.