how to use script to edit Mask?

in Simulink, I have blocks in several liberies, in their Mask Editor: "Icon & Ports" they all have a drawing command something like: image(imread([libPath '\abc\' 'xyz'] ))

How can I use a script to change the command? I'm able to select the block in the model by using : set_param('ModelName','CurrentBlock','BlockName') but I tried to set_param('BlockName','param','value'), it gives me an error: No block diagram 'BlockName' is loaded.

Please advice. Thanks

[zdong]

Sorry I'm a simulink beginner, trying to dig all the info out. Now I got the solution for the error "No block Diagram 'BlockName' is loaded", I have to use : set_param('ModelName/BlockName','param','value')

and the "Icon & Ports" is represented by the parameter 'MaskDisplay'

Thanks