How can I get noised images?

I am studying your paper about despeckling and I want to know how do you add speckle noise to the training dataset. 
For a L-looks image X, I just use N = np.random.gammma(L, 1/L, size) to get a noise picture and get a noised image Y = N*X.  Then, I set the pixel value above 255 to 255.
How did you do it?
thanks!
