http://cg.tutsplus.com/tutorials/luxology-modo/quick-tip-adding-decals-to-your-mxm-materials-in-maxwell-render/

In this Quick Tip tutorial you’ll get a look at something I get asked about a lot – how to add decals/graphics to your materials in Maxwell Render. We’ll be covering two different ways of adding them to your material utilising weight-map textures within the layer groups themselves.