Thanks to NationBuilder's embed.ly tool integration, adding a photo album to your pages and blog posts is as easy as pasting a link - really.
Using the embed.ly tool
Copy a link to any set, slideshow, or image from Flickr, Picasa, or any other supported service.
Next, Paste your link directly into your Content Editor Pane.
NationBuilder recognizes them as content URL's and automatically converts them into embeds.
If you encounter problems getting the embed to show, first make sure you are not using a secure link. If the URL you copy starts https:// that indicates a secure connection to the content. Try removing the "s" from the link, so that the URL begins http://
Also, make sure your link is not wrapped in <p></p> tags or <a></a> tags - you don't want to be hyperlinking to this link, just inputting it as text into the content editor. You can check this by opening the HTML button in your Content Editor Pane and deleting the tags if you see them on your link.
UPDATE 3/21/17: the way Embed.ly handles Flickr slideshows has changed. If you enter a Flickr slideshow URL into the content area of your website as described above, Embed.ly will display the first picture from the slideshow.
You can still embed the full embed code from Flickr under <div id="content"> in your page template.
Flickr slideshows, by default, use Flash to automate their slideshows which will make them incompatible with mobile devices. If you grab the link for a set of pictures from a Flickr account on mobile devices, it may not adjust to the size of the screen and it will become a link to the set on the Flickr website.