Simulations and Anatomical Images


I have a question regarding simulations. What is the preferred program of choice? I am using R, but in order to have my simulation analyzed in FSL or SPM, an anatomical image is to be provided. I’m not really understanding the concept of this anatomical image when it comes to the simulations part. Can an anatomical image be simulated, or must we always have some real anat image on-hand in order to do simulations? Thanks.