This looks like the double variety 'Flore Pleno', since it has more than four petals. Could the image be moved?

Strictly speaking, the photo isn't in the wrong place now. A photo of any plant of this species can be added to the species entry. The plant in this picture looks a little different from the image plantrob added to the 'Flore Pleno' entry. If there's any doubt that it's 'Flore Pleno,' it shouldn't be moved to that entry.

I doubt the cultivar has a rigorous definition. Probably any double form counts, no matter how many petal-like stamens there are. It's a naturally occurring form, apparently, and so probably pretty variable. However, not sure how to verify this.

But look at the synonym listed in the species entry.

Celandine (Chelidonium majus)

I'd be more likely to argue that the 'Flore Pleno' entry should be merged with the species entry. Is 'Flore Pleno' a valid cultivar name or is it simply another way of saying Chelidonium majus var. plenum, which means double-flowered and is now only a synonym for Chelidonium majus?