You can also see the same on the barriers surrounding the building of Nelson's Column here. George Sala would later recall of this very period:
"Trafalgar Square was then being laid out, and the area was surrounded by an immense hoarding, which, notwithstanding minatory notices of "Stick no Bills," and "Bill-Stickers, Beware," was continually plastered over with placards relating to all kinds of things, theatrical and commercial, and at election time with political squibs. There were in those days no bill poster advertising-contractors. The bill-stickers were an independent race, whose main objects in life were first, to get a sufficient number of bills to stick up, and next, to cover over the placards pasted on the hoardings by their rivals. Thus the perpetually superposed bills led to a most amusing confusion of incongruities. If you tried to read, say, six square yards of posters, the information was conveyed to your mind that Madam Malibran was about to appear in the opera of Cockle's Pills; that the leader for Westminster was the only cure for rheumatism that Mr. Van Amburg and his lions would be present at the ball of the Royal Caledonian Asylum; and that the Sun evening newspaper would contain Rowland's Maccassar Oil, two hundred bricks to be sold at a bargain; and the band of the Second Life Guards would be sure to ask for Dunn's penny chocolate at the Philharmonic Concert, with Mademoiselle Duvernay in the Cachuca."
My impression is that such flyposting was cracked-down upon later in the period (note the Sala piece mentions a system of legitimate contractors). I have this, however, from Dickens Dictionary of London in 1879:
Bill-posting —The ordinary charge for hoardings is from a penny to twopence per sheet of “double crown” or “ double demy,” but very great judgment is required both in selecting stations and composing the bill itself. One chief point to bear in mind is to have as little in your bill as possible. Another is to have something novel and striking to the eye. All the best stations are in private hands, and must be treated for in detail. Be careful in all cases to have a written agreement. “Fly posting” – ie. Bills placed broadcast on unprotected stations – may be done very cheaply.
which contrasts 'bill-posting' and 'fly posting' and perhaps suggests that the trade still flourished. The Parry painting is marvellous, regardless, as a piece of social history ... does anyone know of anything similar from later in the century?