The image shows a 121,000 km square region at the center of the Sun's disk. About 100 images of the Earth could be placed in the area shown in the poster. The solar surface consists mostly of an irregular cellular pattern caused by the temperature variations in the surface. The cells, called granules, are evidence of convection, the mechanism that transports heat to the surface. Each of the cells is about the size of Texas.
Convection is the technical term for heat transport by overturning fluid or gas.
This image only captures an instant in time. The surface of the Sun, like the surface of a pot of boiling water, is constantly changing. A picture taken 10 minutes later would look very similar, but it would be hard to recognize any granules common to both images. This occurs because granules only survive for about 5 minutes. The tiny bright points seen throughout the image are sites of magnetic field.