There is really nothing to stop book covers from being as explicit as is physically possible. Erotic romance paperback tend to limit themselves to suggestive silhouette or sliver of breast, back or thigh. These covers are, after all, in the public eye and so genitalia and the female nipple are pretty much verbotten.
Ebooks are on a more informal system. They may go as far as both butt cheeks or a breast in profile ir covered by a hand, seashell or a few locks of flaxen photoshopped hair. But a stroll though All Romance Ebooks new Omnilit store (not so omni, so far) does suggest that more covers are now going the full breast-and-nipple.
How far away is the full frontal? And does it matter? RWA's attempt to impose cover standards was unnecessarily prudish. But a website with no age restriction is in many ways subject to general public acceptance (or at least they want a lot of customers). So how far, for a general ebook site, would be too far? (Or would any kind of limitation be pointless, patronising and draconian?)
p.s. the transformation is complete. Erotica is listed only as a subset of romance.