Wrong! Wrong, wrong, wrong. Take another look at the picture. In that pose, bent over as she is, and in those shoes--if you smacked her bottom right now she'd tip over and fall on her head. Her balance point is way too small. She needs to widen her stance (no Larry Craig jokes, please!). Hands on her ankles will leave her overbalanced. Hands on knees will suffice, and will provide a more stable position for her. Or--even better--perhaps you could move a chair in front of her and let her grip the seat, or place her palms flat on it.
Lastly, one other thing you could do as a spanker is to not stand back at a distance. Tuck her under your arm and wrap it around her waist. Move your leg against hers so that, if necessary, she can lean against you and not topple. You want to spank her, not knock her over.
The safety of the spankee should always be your main concern. The scenario that you played out in your head might sound good, but always be sure to check it against reality and make things as safe as possible. The only thing she should get out of the encounter is a bright, shiny and stinging behind.
The picture below has it right. She still needs to move her feet further apart--and, of course, that skirt needs to come up--but with that chair in front of her for support, the spanker should be able to swing away as hard as he--or the spankee--might like.
Oh, and one other thing......get that yappy little dog out of there......