Can you show 20 RANDOM photos with a tag – rather than the 20 most RECENT photos with that tag?
This is not possible with the current version as Slickr Flicks uses a RSS feed that returns the most recent 20 photos with the chosen tag.
All you can do is display those 20 photos in a slideshow in a random order.
In a later release we will be able to get more photos than 20 and in that case it would be possible to say fetch 50 or a 100 photos and then choose and display 20 at random. The slight down side of this approach is having to fetch more than you need but with caching of the RSS results it should probably perform okay. I expect to have this ready in early August.
This feature was released in July 2011 for Slickr Flickr Pro users