St Salveur capitol of Ile d'Yeu which has the largest concentration of Megaliths anywhere in France.
This is an interesting island, Roscoe, about which I've been trying to find out as much as I can since you referred to it. I'd just note a typo in that the main town is Saint Sauveur, not Saint Salveur. It appears to be covered in megaliths, as you say (like the island of Bornholm), although there's relatively little in the way of individual information on them, other than on what seem to be the two most celebrated sites - the "Bread Henge" stone circle on the south coast, and the Three Dolmens burial chamber on the north coast, which has a French Ministry of Culture listing (ref. PA00110142), and has been a protected monument since 1889.
The Christianised menhir you posted a photo of is the Menhir de Ker Doucet, located according to the Megalithic Portal at Lat 46.711100N and Long 2.3503W. The list below shows the prehistoric sites on the island with distances and co-ordinates relative to this menhir. I don't know how exhaustive a list it is. The Three Dolmens site I referred to above is called La Planche a Puare on this list. I've put the link below - the ones on the list with an asterisk against them have photos attached, but there is little other detail given. If you're looking for alignments, I would have thought the individual standing stones would be the most promising way forward.
472m N 12° Dolmen de Ker Chalon Burial Chamber (Dolmen)
680m S 187° Chien-a-l'Affut Rock Art
959m S 158° Roche aux Fras Rock Art
1.7km W 258° Menhir de Chiron Lazare Standing Stone (Menhir)
1.9km W 249° Cromlech de Barbe Stone Circle
1.9km SE 143° Dolmen des Landes (Ile d'Yeu) Burial Chamber (Dolmen)
2.0km SE 153° Menhir du Pissot Standing Stone (Menhir)
2.2km SE 129° La Guette Chambered Tomb
2.4km SE 154° Pierre Tremblante Burial Chamber (Dolmen)
2.7km SE 119° Menhir de Vrimonière Standing Stone (Menhir)
2.8km NW 304° La Table de Ker Difouaine* Sculptured Stone
2.9km SE 128° Pierres du Pain et du Beurre* Standing Stones
3.0km SE 125° Menhir de Soux* Standing Stone (Menhir)
3.0km NW 295° Dolmen des Tabernaudes* Burial Chamber (Dolmen)
3.2km NW 305° La Gournaise* Burial Chamber (Dolmen)
3.5km W 292° La Planche a Puare* Burial Chamber (Dolmen)http://www.megalithic.co.uk/article.php?sid=6333923
Incidentally, a small thing, but I found it of interest when I saw it on the Wiki link to the old castle - Le Vieux-Château de l'Île d'Yeu. Georges Remi (aka Herge) is said to have used this castle as his inspiration for the Scottish one featured in "The Black Island", one of the early Tintin adventures, which originally appeared in 1937-38.http://en.wikipedia.org/wiki/Vieux-ch%C ... le_d%27Yeuhttp://en.wikipedia.org/wiki/The_Black_Island