Extending still images

When you create a background for animation, it's often necessary that a still image remain the same for several frames. Adding a span of new frames (not keyframes) to a layer extends the contents of the last keyframe in all the new frames.

To extend a still image through multiple frames:

Create an image in the first keyframe of the sequence.
Select a frame to the right, marking the end of the span of frames that you want to add.
Select Insert > Timeline > Frame.

To use a shortcut to extend still images:

Create an image in the first keyframe.
Alt-drag (Windows) or Option-drag (Macintosh) the keyframe to the right. This creates a span of new frames, but without a new keyframe at the end point.