I like this - aesthetically pleasing and the design does not distract from the artists work. My only gripe is that there are next to no cues on the painting detail view that indicate that it is part of a gallery. Once I figured this out it the site was fluid. Would suggest:
Adding indicators/arrows/dots or even a little instruction motion once the user enters the detail view
On desktop some indication that they keyboard can be used would be great too as the site is nice to use this way.
Once inside the painting detail view and at the end of the current gallery why not link to the next gallery? This way someone could flick through all of the works in one view.
A nice to have would be a pull up inside the painting detail view (or similar) to display the gallery abstract information? For someone who enters the gallery past this first slide of abstract information, this would be a good way of making this obvious and accessible.