In recent years it seems we have seen an awful lot of Gerhard Richter. There have been three major exhibitions in London well within the last seven or eight years. One is hardly complaining, since there is always a demand to see “the world’s most influential living painter”, as he is often claimed to be (and not without some reason).

Ex-voto paintings are a tradition in Mexico, an offering of gratitude to God and the saints for answered prayers, row after row of them lining the walls of Mexican churches, testifying to the congregations’ devotion, and to the enduring link between man and the spiritual. In artistic terms, however, these paintings, which have been created from the 16th century onwards, are one of the great national archives of folk art, extraordinary depictions of ordinary people: a moving, comic, tragic, epic narrative that is not to be missed.

Dogs, horses, cows, sheep, goats and pigs are the creatures that, however minuscule in stature, take pride of place in the fascinating exhibition of Thomas Gainsborough’s imaginary landscapes at the Holburne in Bath, an ideal complement to the nine major Gainsborough portraits in their British picture gallery.

Surprisingly for one of the most prominent portrait-painters in all of British art, Gainsborough's animals, lovingly portrayed, their body language based on acute observation, dominate their human counterparts in these landscapes, who are more or less rural stereotypes.

Art about art is one of my favourite kinds of art. Paintings, drawings, sculptures, films - works of art which talk about what art is, what the image is, what art can represent and what it can't - all appeal. It is not just a picture of some prostitutes and some African masks - it is Les demoiselles d'Avignon by Picasso and it blows apart the boundaries of painting by cramming three dimensions into two. And then there is Frank Stella, in a new survey of his career at Haunch of Venison, the ultimate modern artist-about-art - and I'm left cold.

"The sheer adventure and life of the touch is the only relevancy," wrote Barry Flanagan in his graduation thesis for St Martin’s School of Art in 1966. "I must allow my hand to touch and feel, my eyes to look and see, my tongue to lick and taste, my nose to sniff and smell, my ears to listen and hear."

A 50^th birthday is a landmark occasion. One has plenty to look back on, whilst still having much to look forward to. Plus there’s all that life experience to draw on. What’s not to feel positive about? In the case of a gallery that’s built up a remarkable reputation as an innovative space for contemporary art outside London, sheer staying power is surely to be cheered and celebrated – "hear hear" for the next 50 years, and so forth.

It took a long time for architects to embrace popular culture. I attended a talk at the Architectural Association in the mid 1970s, when someone (probably the architect Robert Venturi) waxed lyrical about shiny American diners and hot-dog stands shaped like Frankfurters and extolled the virtues of the madcap fantasies built in Las Vegas.

John Martin is heaven. Well, as many of his contemporaries would have pointed out, John Martin is also hell, or The Last Judgement, or, as the Tate’s show title would have it, the Apocalypse at the very least. For John Martin was, after Turner, the 19th century’s premier painter of catastrophe. Unlike Turner, however, he was not much rated by the more respected critics, and his work, frequently oversized, tends to spend more time in storage than on the walls of public galleries.

A beguiling shadow play greets and enchants on arrival: the silhouettes of three ballerinas, each performing an arabesque, are cast upon the wall as you enter. The effect, as their softly delineated forms dip and slowly rotate, is mesmerising. It’s also an apt opener to an exhibition devoted to exploring how Degas strove to achieve a sense of fluidity and movement in his paintings of dancers, a subject for which he is chiefly known.

A retrospective of an artist’s work is not usually a history of a working relationship, but in the case of Christo, this impressive exhibition of works from the past 40 years also marks two crucial partnerships: with his wife, Jeanne-Claude, who was his equal and co-creator from 1961, and with the Annely Juda gallery, which has mounted 12 exhibitions over four decades, as well as being intimately involved in their massive environmental “wrapped” pieces. Photographs of the end results are breathtaking, but even more gripping is watching the development of the processes over the years.