Scaling down a raster image
Bilinear is the most widely used method. The nearest neighbor down-sampling algorithm is the fastest but least accurate. 

Note that when you are trying to down-scale an image by half, the bilinear sampling algorithm becomes the average (mean as you mentioned):

x1[i/2][j/2] = (x[i][j] + x[i+1][j] + x[i][j+1] + x[i+1][j+1]) / 4;