Writing about paintings

This is a narrative rather than a how-to; as I have implied in earlier posts, I'm teaching the class that I am in part to teach myself how to do this. This is gonna appear pretty trite, especially to the more knowledgeable among you, so be forewarned.
I've told my students that I will be doing the same writing assignments that they are; so far, on the class's homepage I've posted a physical description of Rothko's Brown and Blue and Brown on Blue (disclaimer: the lower band in the original is NOT green, much less THAT green), and I made extensive notes on a painting I saw in the Wichita Art Museum on Saturday, John LaFarge's Autumn (sorry, no online image that I can find). I have to say that in both cases, this has been some of the most challenging writing I've ever done--and I'm not talking about interpretation (I've not begun to do that in writing for the Rothko, though I have some ideas about how to proceed) but, rather, merely describing the physical appearance of the paintings: the colors, textures, the pose of the woman in the LaFarge, the gradations of colors in the Rothko. It seems paradoxical, doesn't it: the apparently simple act of describing what one SEES. But, at least with me, I found that the more I looked, the more I saw.
The LaFarge composition, for example, is very simple: a woman dominates the painting, holding a woven basket, some indeterminate foliage behind her. I literally wouldn't have given it a second look, though, if it had not been for her odd posture: she is bent at the knees and waist as though her basket is filled, but her hands don't grip the basket--instead, they are sort of pressed to either side. That fact, and the fact that her body is turned toward the painting's light source but her face is turned away from it, kept me busy describing (and interpreting) for more than an hour. Exhausting but exhilarating.
Let's be clear: the LaFarge is a minor painting in the grand scheme of things, even among his own work. But its oddness certainly kept me much busier than I would have guessed.
So: John Berger and James Elkins (not to mention my painter friend Susan), each in his/her own way, are right: one has to begin with the image itself, noticing everything, unfiltered by what one knows or has heard about the painter and forgetting, as Berger would argue, that one is seeing something enshrined in a museum. Yesterday I read something Cezanne wrote to another painter, and it seems appropriate here (though today, dammit, I can't see to find it so I'll have to quote inaccurately): "Museums are like Plato's cave. I would banish all artists from them. There is plenty of sunlight outside." Something like that. For Cezanne, ultimately what one should paint is what is in nature, what is THERE, unencumbered by What Others Have Painted. The same, by analogy, seems to be true for writing about paintings: begin with the image, THEN with why one thinks the artist chose to render the image as s/he did.
Where else to start, except with one's own impressions?