Raster

Raster[{{a11,a12,…},…}] is a two-dimensional graphics primitive which represents a rectangular array of gray cells. 

Raster[{{{r11,g11,b11},…},…}] represents an array of RGB color cells. 

Raster[{{{r11,g11,b11,α11},…},…}] represents an array of color cells with opacity αij. 

Raster[{{{a11,α11},…},…}] represents an array of gray cells with the specified opacities. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.