Here is the public query string:
http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20tumblr.posts%20where%20username%3D'staff'%20&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys
Of course, "public" needs to be changed to "yql" but where do you add API key info?