For region attachments, the origin is the center of the images. You could use a mesh, which allows the origin to be moved. To do that, hover the crosshair until it changes to a circle, then you can move it (here for rotate, but it works for any tool):
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Or you could place a bone where you want the origin for scaling, then scale the bone. This is most common, since you can animate the bone. You can't animate the scale of a region attachment. You can animate mesh vertices (via deform keys) but it requires a lot more data and is much less efficient than using a bone.