You can add images as icons on a Simulink mask and save them in the SLX file.
Select any masked block and on the Block tab, click Add Image.
In the Add mask icon image dialog box, click
Browse to select an image from your local repository. You can also
set the transparency using the Icon transparency field. Available
Opaque with ports.
Select the Store a copy of the image in the SLX file check box if you want to store the mask image in the SLX file.
You cannot store mask images in an MDL file.
Click OK to save your changes.
Convert mask image to internal for one block
Convert mask image to internal for the whole model
Convert mask image to external for one block