If you only care about supporting the Image material so you can apply a texture to each face of the cube, the fragment shader can be as simple as
varying vec2 v_textureCoordinates;
materialInput.st = v_textureCoordinates;
czm_material material = czm_getMaterial(materialInput);
gl_FragColor = vec4(material.diffuse, material.alpha);
This is not a general purpose shader, and will only work with a few materials. You'll also need to pass v_textureCoordinates in from the vertex shader, e.g., see [ViewportQuadVS.glsl](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Shaders/ViewportQuadVS.glsl), and compute the texture coordinates for each face of the cube (0 to 1 in each direction) that are provided to the vertex shader. For example, see [ViewportQuad.js](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/ViewportQuad.js).