This is "Brain Jag" (1987) by Rob Chilson, and it appears to have only been published in Analog, June 1987.
My memory of the art is confused with another story, but it does show an artist at an easel. My memory of the setup of the story is also incorrect; Professor-Emeritus Morea is called in by the court to be an expert witness in the case where Lindine Polk stole Jesus Sanchez's "pocket brain" and used it to paint a picture.
Professor-Emeritus Allen Morea looked wryly back. "I have here a court order - I'm supposed to be some kind of tom-fool witness in the case-"
"[...]This Lindine Polk stole another artist's pocket brain, is that it?"
"So they tell me. That is, the other artist, what's his name - Jesus Sanchez - says he did. Polk claims he just borrowed the confounded thing."
The bit about the painting is right, though:
It was a high-key piece, the colors intense: lush greenery, mountains dramatic in the background, the ruins of a Mexican step-pyramid in the middle distance. The brush-strokes were loose, bold, vigorous; the flowers were impressionistic splashes of color.
There was a nice effect with one bright red bird, which was merely glimpsed as a pair of brush-strokes, a blur of motion, superimposed on the greenery and the distant mountain. This was the center and focal point of the picture.
Sanchez's work has a lot of scenery/background elements similar to the painting in question, but his portrayal of people and animals is stiff:
The birds seemed stiff and poised, painfully so when flying by.
Morea muses disparagingly of art done by people wearing pocket brains:
Morea repressed a snort of contempt; it was all, in his opinion, style over substance. If art doesn't have feeling it isn't art. At best it's illustration - fine in its way - but...
Morea also doesn't think too highly (though better) of Polk's work:
In general, Polk's inability to illuminate his human model's natures - or his own - often made his work pointless, ultimately boring.
Morea perceives that Polk's work was improved by Sanchez's pocket brain:
As he said, they were visibly in his style, but to Morea's surprise they were not as pointless as usual with him. Of course, he told himself unhappily, one can't generalize from three sketches... yet, there was an undeniable verve, force and vigor here that echoed the best things of Sanchez's paintings.
Morea actually gets upset about the idea of copying the personality overlays developed by the pocket brain and selling them, thus allowing anyone to use the skills of a talented person:
The personality-analogue would be treated by the pocket brain like any other program. Why, it would be a simply mater to have it pull the analogue out and store it - or copy it - on a bubble. And pneumobubbles could be duplicated with all their data, by the thousands, millions, for a buck or so apiece.
Paint like Picasso! Blow your horn like Boots Randolph! Write like Wrede!
It would be literally possible to sell your soul - if you were famous - to the multitude. Singers, rock stars, cloning themselves on the hapless followers. My God, think of all the books "in the tradition of" Tolkien. But of all the living
artists, writers - oh my God! Actors. How can they ever acquire characters, personalities of their own, if they can buy those of great actors and actresses for a song?
He gets so mad and worked up he puts his boot through the painting:
In his anger and despair Morea lashed out blindly, felt the shock in his ankle and leg, and heard the rending thump of canvas.
He'd kicked a hole right through the picture.
I found the page PDAs in Science Fiction, and was working through it when I found the entry "1986-1986 - Rob Chilson The "Pocket Brain" series."; googling
Rob Chilson "pocket brain" took me to a couple of machine-translated, unattributed copies of this question, but while the question sounded similar, the answer didn't really give me more information than the "PDAs in Science Fiction" page. Fortunately I finally managed to dig up the relevant issues of Analog and figured out the actual story in question. I may actually go and write a better answer for that question, since it's obviously specific to the first story.