Hopefully fixed for the next release.
This is a case of the images not really aligning with the variables. Due to Penny having so many image variants (with a couple not even in a complete set, WolfyNail's art included), some images were just not being unlocked because the variant is unreachable. With respect to the WolfyNail art, it was labeled as "penny_bimbo" and that is never true as the bimbo version of Penny always as a penis type ("_equine" or "_vixen", plus various size variants). I've reviewed the content and updated WolfyNail's bust art to only be available for "penny_bimbo_vixen" (the smallest vulpine penis with bimbo variant), so if you have that already unlocked, it should show up in the gallery. And to avoid confusion, I've also hidden any other "penny_bimbo" art from the gallery, as they are inaccessible.
Do note that this is the same image as the CG image used for when Penny first sprouts a penis via using Throbb, just cropped and with a transparent background. I am aware there is a mismatch between the hairstyle for the bimbo and regular Penny, but the Flash version of the game used this bust particularly for the bimbo variation (possibly due to the characer being half-clothed). So despite that inconsistency, it should match up with the intended use for the bust art.