Go to your Photos at Flickr.com >Go to the Image >'Share This' > 'Grab the Html'
Embed the Image URL directly in Google Sites.
Go to your Photos at Flickr.com > 'Slideshow' > 'Share' > 'Grab the Embeded Html'
You will need to modify the Slideshow size (width and Height) Twice! in the flickr Html code first, before pasting into your Html. The Google Sites Properties > Sizes, will have no Effect on the Slideshow.