Using Photocasts
Photocasts are a way of distributing images using RSS feeds. They are typically used by photo-sharing sites such as Flickr; you can subscribe to a friend's feed using CODA and you will see a selection of their most-recently-posted images which will update over time as they post new ones.
Different sites will call these feeds different things. If you go to somebody's main page on Flickr (eg. Quentin's), you'll often find a link near the bottom of the page to a 'Photostream' of that person's latest images. That's the URL you want to use when creating your photocast in CODA.
Blogs which are mostly text but incorporate some photos should be created as RSS Feeds. When a feed is purely images, though, and you want to view it as a slideshow, that's when you use a Photocast.
It's worth noting, though, that some image feeds only include fairly low-resolution pictures; these won't look so good when scaled up on a large display.