Summarize:

You'll need to locate it on your computer. Use the Ctrl + Select combination to select multiple. Drag them across the screen and drop them onto the text field where you write your post on the Facebook page. Write anything about it or tag a friend. When you’re done, click on “Post” button to share the pictures.
Open the folder containing your photos. Select the pictures you want to share. Drag the selected photos to your Facebook. Wait for the image to load and show below the text field. Share your photos.