To save a picture as an Image Snippet:

Choose Images from the popup menu beneath the light blue source list on the left of the window, or press Command Option I.

Click the + button in the bottom left corner of the window.

Drag the image you want to save to the picture icon in the dark blue-gray area near the bottom left of the window.

Type a name for the Image Snippet in the text field beneath the Description label in the dark blue-gray area near the bottom left of the window.