I can't imagine a more dynamic and difficult subject: children, fabric, moving water, animals, and boats in the surf, contre-jour lighting, and probably sand, spectators, and worst of all, wind. All in a day's work for Sr. Sorolla.
He's working on a folding wooden tripod easel that was pretty typical for his times. His palette is resting on a low folding chair or table to his left. Even with the wide stance of the tripod, a gust of wind would blow this thing over. He always has a nice suit of clothes, good shoes, and a different hat.
The palette is in the left hand, and there's a chair on the right with the paint and brushes easy to reach. There's probably a farmer out of frame with a bowl of scraps doled out slowly to keep the pigs in place.
He's sitting this time, which lowers the center of windage. The paint box is on the chair at right. If that's an assistant, he seems to be holding another chair.
The kids are taking turns as models. There seems to be a weight hanging from the easel to stabilize it, and the top of the panting appears to be resting against the rope. Judging from the fabric bellying out at right, the wind is a factor.
Now he's working much larger. The stretched canvas is mounted on a hefty wooden base structure, perhaps with some wood pieces driven down into the sand. The ladder/scaffold lets him reach higher in the picture. A couple of assistants are there to help.
Here's what he is working on. Even with models for the kid and the horse to look at, there's a lot of memory work involved here.
Now he's working on the epic mural project on the peoples of Spain. He has enlisted local models to pose outdoors in costume. The large canvas is held vertical with weighted diagonals, and the base of the canvas is about a foot off the ground.
In his studio, he often has his palette on a low table and used long brushes to be able to paint with a full arm reach, backing up as far as he could to compare the painting to the model.
Here he's painting in the garden of the manor "Vista Alegre." He has portable stairs to stand on and a wooden box for his paints. There's a box-like structure built around the whole gigantic painting, and some shear fabric held up on both sides, which was described as "an awning to protect the paint."
It looks like a set-up that he could leave deployed for a while. An observer recalled seeing "the construction of a large boardwalk outdoors where he could install his paintings and a scaffold to support the frame weight." The models were employees of the estate, and he also needed to hire a translator because he had difficulty understanding Galician. (Read more about this on a Spanish website.)
The big painting seen in the photo is "Galicia," one of the murals from the Hispanic Society in New York.
Book: Sorolla: The Masterworks