It looks like jquery populates the "_" parameter with a timestamp to defeat any request-caching, and google/picasa does not like extra parameters in its requests. As a workaround, you can disable this by setting jQuery.ajaxSettings.cache = true before calling $.getJSON, and restore its original value after the call if desired.
I am pretty sure that the jsonp callback name which is generated for anonymous functions (i.e. those passed inline to the getJSON call) is unique, so you should not see any caching of the requests even with the "_" parameter removed. - MF
