Hi there, jhillemann! Thanks for using the Apple Support Communities. From your post, I understand that you want to keep the Photos app in your Dock. I'd be happy to help make sure that happens! In Sierra, the Photos app is in the Dock by default. If it's been removed accidentally, you can add it back by going into Launchpad or the Applications folder, locating the Photos app, and dragging it to the Dock. More information on doing this can be found here: Mac Basics: The Dock holds your favorite apps, documents, and more Additionally, any open app will appear in the Dock, whether you keep it there permanently or not. Open apps appear with a dot underneath them in the Dock. Have a great day!
Bonus: Creating a Folder of Apps in the Dock. What if, apart from adding spaces to organize your apps in the Dock, you also want to create a folder of apps. Creating a folder should organize things, as it will declutter your Mac and allow you to keep more apps inside the Dock. MacOS doesn’t allow you to create a folder of apps in the directly. Adding a single App icon to dock is very simple, you just need to drag and drop in the free space of the dock. Removing an app from dock also easy, you need to drag the app icon from dock and drop into the Trash (Recycle bin). Always, grouping and categorizing apps in Mac OS X is good and useful, because you can access your most frequent apps. For future looking for a programme not in the dock, you can just use Spotlight (CMD + Space bar) and typing in the name of the software. Normally it is located under Applications (in the sidebar). And you can drag the Applications folder to the dock if it isn't already there. The Dock on your Mac. The Dock, at the bottom of the screen, is a convenient place to keep the apps and documents you use frequently. Right clicking on the Application's Dock icon, highlight over options, and unselect 'Keep in Dock'. You'll need to do this for all the Applications in Dock. If you wish to have ONLY open applications in the Dock, here is a Terminal.app command you can run to change the preferences to show only open applications in the Dock. Mac os firewall incoming connection app master.