Now that your images are ready to use, you can simply start dragging them from the file palette  onto the “canvas” in the center of your screen to begin assembling the first key frame.  Once on the canvas, you can select any Sprite (the image's you've placed) by left clicking, and then use the transform controls which appear around the sprite to rotate it or stretch it as you need.  You can also use the “object properties” dialogue on the lower left of your screen to keep track of or carefully edit any of the  currently selected sprites attributes. This palette can also be used to adjust the currently selected sprites opacity.
Adding Sprites To the Canvas (frame)

Spriter Pro User’s Manual version 1.4