In Aldia's Keep on NG+, you find the Painting Protector. They wear the Monastery Set, a light armor very similar to the Painting Guardian set from Dark Souls. They even drop the same weapon as well. The set itself mentions two things. The first is that the set is from the Lindelt Monastery. The second, is that "they revered whatever it was they watched over". I believe, that the two sets are in fact from the same cult that guarded the Great Painting of Ariamis in Lordran.

Lindelt is known for being the home of many Clerics, Miracles, and for having the most strindgent of laws. The Miracle Unveil was used by the Monastery to find those that broke the Monastery's rules, and the Archdrake Mace mentions how in Lindelt, "those who dare defy them are punished without mercy." 

We also know that both the Soothing Sunlight and Bountiful Sunlight Miracles were once kept at Lindelt, but were stolen and never recovered. Licia possesses the Soothing Sunlight Miracle, and you must kill her to take it from her. The Bountiful Sunlight Miracle is granted by the Blue Sentinel Targray. Licia, likely being the Nameless Usurper as well as a conartist, probably stole at least one of the Miracles. Targray may have helped her steal the Miracles, but it is more likely that he recovered the Bountiful Sunlight Miracle from Licia. It is possible that he even chased her to Drangleic to retrieve the Miracles. In either case, both of the Miracles were exclusive to the Princess Guard Covenant, and were the Miracles of Gwynevere. The Way of the Blue originated from Heide, but has since spread to the rest of the world. According to Maughlin, the Blue Sentinels exist even in Volgen, and heavily govern trade.

So where is all this going? I believe that the Monastery in Lindelt is the resting place of the Great Painting of Ariamis, and that is why the Painting Guardian set is now made there. In fact, I believe that much of Anor Londo found its way into Lindelt. The part that I can actually explain is that most of the Miracles in the game seem to come from or be sold by individuals from Lindelt. The Painting Guardians from Anor Londo now exists in Lindelt, likely due to the presence of the Painting of Ariamis there. The Archdrake sect is hiding something that only the most trusted individuals are allowed to know, and rout out and punish any that do not obey their rules.

Here's what I speculate, and it is heavily speculated. Heide is where the gods of Lordran moved to when Lordran began falling apart. That is why the architecture in Heide is so similar. When Hiede fell into the seas, they moved again to what is now known as Lindelt. They brought with them the Miracles, the Painting, and the Way of Blue/Blue Sentinels, which are basically the new Darkmoon Blade. The secret that the Archdrake sect guards is that the gods still live today in Lindelt, slowly rebuilding their power. The Archdrake Priests found in Drangleic may have been sent to recover the Lordvessel, the First Flame, or even the Miracles stolen from Lindelt.

That is speculation, my main argument though, is that the Painting of Ariamis, and in fact Priscilla, are likely in the Lindelt Monastery.