1. A dump truck periphery monitoring system that monitors a periphery of a dump truck including a vehicle body frame which supports front and rear wheels and a vessel which protrudes toward a rear side of the vehicle body frame than the rear wheels to load freight thereon, the dump truck periphery monitoring system comprising: a plurality of image capturing devices each of which is attached to the dump truck and captures the periphery of the dump truck to output a piece of image information, the image capturing devices including a rear-side image capturing device which is disposed below the vessel, an imaging range of the rear-side image capturing device including a lower area of the vessel; a bird's eye image synthesizing unit that acquires a plurality pieces of the image information and synthesizes the acquired image information to generate bird's eye image information for displaying a bird's eye image of the dump truck and the periphery of the dump truck on a display device; a display control unit that switches, on the display device, a display of one of a first image obtained by superimposing; a first vessel image including the vessel and excluding an entire vessel of a rear-side part of the vessel than an end of the rear wheel; and the bird's eye image in which the first vessel image is positioned at an area the dump truck is present, so that a lower area of a rear portion of the vessel corresponding to and positioned at the rear-side part of the vessel in the first vessel image is displayed as a part of the bird's eye image, and a second image obtained by superimposing a second vessel image including all of the vessel at the area in the bird's eye image; and a determination unit that determines whether a running speed at which the dump truck runs becomes a predetermined threshold value or less, wherein the display control unit displays the first image on the display device when the running speed at which the dump truck runs becomes the predetermined threshold value or less, and displays the second image on the display device when the running speed at which the dump truck runs becomes larger than the predetermined threshold value.