Custom Twitter, Facebook, and Instagram posts can be added to playlist. This can be useful if you want to display an old post and it is too far down to select from a Channel. If you have the permalink to a post you can just pasted it into the field in the "Add Post" dialogue (this is currently only supported for Twitter and Instagram)
You can also create custom posts by filling in the fields in the dialogue. You will need to host the images somewhere public so that when you enter an image URL the system can access it. You can upload these images to a public Dropbox folder, Google Drive, or server where the image can be found with a URL.