That's a practice backstop.

Possibly it would help to know that in London it was not called tennis back then. There were three different sets of events held in the sport we now call tennis. In 1908, "tennis" usually referred to the specific indoor game often called court tennis, real tennis, royal tennis, or jeu de paume, and which was contested at the 1908 Olympics. In those days, "tennis," as we now know it, was usually contested on grass courts, and was termed lawn tennis. However, in addition, an indoor version of the sport was also on the 1908 Olympic program, termed covered court tennis.

The lawn tennis events of the 1908 Olympics were held at that renowned facility, the All-England Lawn Tennis & Croquet Club, better known to the rest of the world as Wimbledon. The covered court events were held at Queen's Club in West Kensington, which also hosted the jeu de paume and racquets events of the 1908 Olympics.

Until Dennis Ralston and John McEnroe, anyway. But never mind that, what's that wooden apparatus behind him?

E. Fischer beat W.C. Grant, 6-3, 6-1 in the preliminary round.

http://www.la84foundation.org/SportsLibrary/Outing/Volume_28/outXXVIII06...

I went searching to see how Mr. Grant did and had no luck in finding him but I did find out that tug of war was an olympic event from 1900 to 1920, and the London team beat Liverpool in 1908.