You can apply bitmap images to your models as texture maps. For example, when you want to apply a texture map that was created using another paint application and then continue painting or sculpting the model using it as a reference while working in Mudbox.
All paint/texture maps are managed using the Layers window.
The image is applied to the assigned material on the model based on the UV texture coordinates and is loaded as a Diffuse paint channel layer. For more information on UV texture coordinates, see UVs overview.