There are site that do allow you to sel photos, adult work and only fans spring to mind but I am sure there are others.
Selling images of naked breasts isn’t illegal (in the UK) but just be careful if you are spicing the image up just double check that what you want to do doesn’t fall foul of extreme porn laws (not all acts on the list are particularly extreme, such as squirting and water sports)
I don’t want to sound like I’m negative about this; if she wants to show off her assets she should go for it. Just be aware that once the image is on the internet, you’ll have very little control of it. Even if you delete the profiles etc, the images may have been saved or screenshot.
I would have a serious look at your partners body. Although she doesn’t want to include her face, tattoos, piercings, moles etc may make her identifiable.