On your Mac
When you use the following import commands, OS X quietly and efficiently adds the files to a waiting folder in the Photos Library package file and, the next time you launch your System Photo Library (the main library that works with iCloud Photo Library), your digital goodies are automatically imported. You'll find them at the end of the All Photos album, though they're also visible in Photos view inside the moment the file chronologically belongs to. Here's how to do it:
Finder. Control-click an item anywhere in the Finder and choose Share > Add to Photos from the resulting shortcut menu or, if the item is in a Finder window, click the Share icon in the window's toolbar and choose Add to Photos. You can also import the file by dragging it atop the Photos icon in your dock, or onto the Photos window itself if it's open.
Mail. If you get an email with a picture or video attached, simply Control-click the item and from the shortcut menu that appears, choose Export to Photos or Share > Add to Photos. You can also drag the item atop the Photos icon in your dock.
Messages. If you receive an image in the Messages app on your Mac, Control-click it and choose Add to Photos Library from the shortcut menu. Alternatively, you can drag the image from the Messages app onto the Photos icon in your dock or atop the Photos window itself.
Safari. If you're on a surfing Safari (ha!) and browsing a webpage or accessing your webmail, you can save an image on the page or in an email message by Control-clicking it and choosing Add to Photos Library. You can also drag the file onto the Photos icon in your dock.
Photo Booth. If you capture an image using the Photo Booth app (it's preinstalled on your Mac), select the image and then click the Share icon and choose Add to Photos. Or, simply drag it onto the Photos icon in your dock or onto the Photos window itself.
Preview. If you open an image in the Preview app, click the Share icon in Preview's toolbar and choose "Add to Photos".
This trick also lets you import images into Photos from apps that don't sport a share icon. Shockingly, the selfish app list includes Apple's own Pages, Keynote, and TextEdit, as well as Adobe's Acrobat and Reader, and (unsurprisingly) all of the Microsoft Office apps.
From within any program that lets you select an image, click it and then copy it to your Mac's clipboard by choosing Edit > Copy. Launch the Preview app and choose File > New. When you do, the contents of your Mac's clipboard--the image you just copied--is pasted into a new Preview document. Click the Share icon in Preview's toolbar and choose Add to Photos.
This works for importing headshots into Photos from the Contacts app, too. In Contacts, click an image associated with one of your records and choose Edit > Copy and then follow the instructions above. The resulting images are visible at the end of the All Photos album, but you'll find them in Photos view, too, in the moment based on the date and time you created the Preview document.
Scanning into Photos. If you use Apple's Image Capture utility (it's in your Applications folder), you can scan into Photos. Whenever your scanner is plugged into your Mac, or available wirelessly on your network, Image Capture recognizes it and summons its own scanning software, which just happens to contain a Scan To menu that includes Photos (circled here).
On your iOS device
You can also import goodies into Photos for iOS using other iOS apps. Many iOS apps sport their own Share icon that you can use to save the image to your device, which plops it into that device's Photos library. On the other hand, if you're the lucky recipient of a picture or video in the Mail or Messages app, simply tap the file preview and then tap the Save Image icon that appears.
If you're surfing the web using Safari and you come across an image you want to save--say, an image of the couch, car or Man Cave of your dreams--tap and hold your finger down atop the image and from the menu that appears, tap Save Image.
As you can see, there are several ways to get your stuff into Photos. Until next time and may the creative force be with you all!