There are several methods to synthesize fractal images. It depend on data (isotropic or anisotropic). You can find Cholesky, Weierstrass-Mandelbrot,...but for exact data I recommend for you to use the Stein method which is a fast and exact synthesis method for isotropic fBm images.
M. Stein. Fast and exact simulation of fractional Brownian surfaces. Journal of computational and graphical statistics , 11(3):587–599, 2002