The word "ostentation" refers to showy or extravagant displays of wealth, character or other forms of social power, intended to impress others. In keeping with Tom's character (in that he is a person of poor integrity and class) his money is able to buy him power, but not taste, and he feels that he needs to make a show of things by buying stuff that looks impressive, such as a large home. However, he is a miser at heart, and so he barely furnishes this home, because its purpose is largely for show anyway.
The narrator has a knowing tone about this; he says Tom bought the house "as usual", indicating that Tom is cut from the same cloth as any number of other petty self-congratulating types, and that this behavior is to be expected. The word ostentation is referred to specifically as Tom's motivation for buying the house; he needs to be noticed.
So, a good drawing for the word might be an illustration of the home; large and impressive on the outside, but empty on the inside. Ostentation also tends to refer to personal behavior, so an illustration might show a person dressing or acting in a loud, showy manner that is inappropriate for the circumstances, such as being too expensive or too revealing. As a connection to modern culture, elements of many rap music videos are devoted to ostentatious displays of wealth and power, as are some occasions where celebrities or politicians "buy out" an expensive hotel or location and use it for their own business.