Here's a bit more information about model's releases. They basically protect you on the legal "privacy" issues associated with using someone's likeness.
You, of course, own the painting, since she hasn't paid in full for it. And even after a painting is sold, you still own the copyright of the painting, of course.
Model's releases are different. They give you permission to use the person's likeness, either to exhibit any image you painted publicly, to put it on your website, or in any other marketing materials, if you wish.
I include a model's release section in my contract, basically giving me permission to use their likeness as part of my marketing efforts. Without that release, I could be sued, even if I own the painting and own the copyright.
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