"An Artist Painting a Seal of Chastity on His Wife"

A most curious subject matter for a painting; another version (perhaps from the collections of the Hermitage), dated 1732, is on file at Wikipedia.  The painter - Pierre Subleyras - is apparently well known.

He was a remarkably incisive portraitist, as evident from the portrait of Pope Benedict XIV or of the obese Cardinal Valenti Gonzaga. The pope himself commanded two great paintings, the "Marriage of St Catherine" and the "Ecstasy of St Camilla", which he placed in his own private apartments.

Subleyras shows greater individuality in his curious genre pictures, which he produced in considerable number (Louvre). In his illustrations of Fontaine and Boccaccio his true relation to the modern era comes out...

The painting embedded above is from the latter category.  Re the subject matter, I found an indication that the embedded painting is called "The Pack Saddle," with an explanation that the story by Fontaine is of an artist who painted an ass (animal) on his wife as a test of her fidelity, reasoning that any extramarital activity would smudge or erase the painting.  The man who cuckolded him, however, repainted the ass, adding a pack-saddle on the animal.

Perhaps some reader more knowledgeable on classic literature or art can clarify the details.

Addendum:  A hat tip to reader mbarnato, who found the relevant text by Jean de La Fontaine.  It is a poem, which in translation is entitled "The Pack Saddle."

A FAMOUS painter, jealous of his wife;
Whose charms he valued more than fame or life,
When going on a journey used his art,
To paint an ASS upon a certain part,
(Umbilical, 'tis said) and like a seal:
Impressive token, nothing thence to steal.

A BROTHER brush, enamoured of the dame;
Now took advantage, and declared his flame:
The Ass effaced, but God knows how 'twas done;
Another soon howe'er he had begun,
And finished well, upon the very spot;
In painting, few more praises ever got;
But want of recollection made him place
A saddle, where before he none could trace.

THE husband, when returned, desired to look
At what he drew, when leave he lately took.
Yes, see my dear, the wily wife replied,
The Ass is witness, faithful I abide.
Zounds! said the painter, when he got a sight,--
What!--you'd persuade me ev'ry thing is right?
I wish the witness you display so well,
And him who saddled it, were both in Hell.

Embedded image found at First Time User.