Do these painting look familiar? I hope so. Do you know where to see them in person? You have to travel to London. But they won't be found in top-of-the-line art museums such as the Tate or the National Gallery. They're in places a notch below the "must see if you're in town for only three days" list. Take heed: they're surrounded by other notable works of art.
The Hals painting is in the Wallace Collection, somewhat off the usual tourist track. The Wallace is a short ways east of Baker Street in the zone between Oxford Street on the south and Madame Tussaud's and the Sherlock Holmes Museum to the north. And there's no Underground stop nearby, so you'll have to blow some pounds for a cab or else hoof it.
The Manet is in the Courtauld Gallery, a smallish museum occupying part of a wing of Somerset House on the Strand. It's south of the Covent Garden area and the Strand itself is probably the route one might take from Trafalgar Square to the City and St. Paul's Cathedral. The nearest Underground station is several blocks away at Temple, on the Embankment (Circle and District lines).