Step 11) (Swapping a sprites Image at any time) You might have a need in some animations to actually swap out one image with another at some point.  This can be done by simply scrolling to the point in the animation you'd like an image changed, selecting the sprite by left clicking on it in the canvas and then right-clicking any image in the file palette.  When you play the animation or scroll through the time-line you'll see that the sprite now changes from its original images to the new one you'd selected at that exact moment in the animation.
IMPORTANT: When swapping, a new image will be placed in the exact location of the current image based on their respective pivot points, so it's very important to have the default pivot-point of the new image set appropriately.  One way to ensure perfect continuity when swapping from one image to the next is to actually insure that each image to be swapped has the same exact pixel-dimensions as each other, and are arranged so that all of their default pivot points would have the same exact pixel coordinate.  Look at all these head images as an example... all images are buffered with enough transparent pixels to ensure the actual head images are placed per image in perfect alignment with each other... so the exact same default pivot-point coordinates can be set for all...ensuring there will be no jarring “pop” in position as one image is switched to the next.
Swapping The Image of a Sprite

Spriter Pro User’s Manual version 1.4