For Hypershade, create 32x32 icons in xpm format. (You can use imconvert to convert images to xpm.) The icon name must have the preface "render_". For example, for the shader lambertShader.mll, name the icon render_lambertShader.xpm.

Put the icons in one of the directories specified in your XBMLANGPATH. You can extend this path by modifying maya.env. Type "getenv XBMLANGPATH" in the Maya script editor to see the current setting for the path.