For the Create bar, create 32x32 icons in PNG format. The icon name must have the preface "render_". For example, for the shader lambertShader.mll, name the icon render_lambertShader.png. You can also supply an icon for the Hypershade graph. This icon can be in SVG or PNG format, and the icon name and the shader name should match. For example, for the shader lambertShader.mll, name the icon lambertShader.svg.

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.