ImageAdd
[image, $expr_1$, $expr_2$, ...]i = Image[{{0, 0.5, 0.2, 0.1, 0.9}, {1.0, 0.1, 0.3, 0.8, 0.6}}];
ImageAdd[i, 0.5]
ImageAdd[i, i]
ein = Import["ExampleData/Einstein.jpg"];
noise = RandomImage[{-0.1, 0.1}, ImageDimensions[ein]];
ImageAdd[noise, ein]
hedy = Import["ExampleData/hedy.tif"];
noise = RandomImage[{-0.2, 0.2}, ImageDimensions[hedy], ColorSpace -> "RGB"];
ImageAdd[noise, hedy]