Can I use the feeds of a particular Tag?
Yes, and this can even serve as an effective solution to the shortcomings of the set feeds (as mentioned above). You can apply a specific tag to each photo in a set, and then use the tag feed with the widget. While viewing a particular tag, scroll down and click the “Latest” link in the RSS area. The tag feed will appear in your address bar. Copy and paste it into the Flickr widget to display only photos with that exact tag. http://en.support.wordpress.com/widgets/flickr-widget/