The Comtesse d’Egmont Pignatelli in Spanish Costume

The Comtesse d’Egmont Pignatelli in Spanish Costume (1763).
Alexander Roslin (Swedish, 1718–98).

The dress! Just look at that glorious dress! I could look at it all day long.

This is via Books and Art where they said this about the painting

The countess’s glowing gown of white satin has sleeves slashed and woven through with ribbons and pearls, in the Spanish style, a reference to her husband’s ancestry. A talented musician, the countess played the guitar for her Spanish guests. She spent her days studying history and literature and conversing with artists and poets. The book she is holding may be a work by the philosopher and writer Jean-Jacques Rousseau.