Flickr Photo Gallery Embed:

Flickr Embed Image:

Flickr Images seem Better Quailty less compressed than Picasa and Photobucket.

Go to your Photos at >Go to the Image >'Share This' > 'Grab the Html'

Embed the Image URL directly in Google Sites.

Flickr Embed Slideshow:

Go to your Photos at > '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.