How to set the Image path to the ApperanceAssetElement of a Material

Dear all,

      I am developing a function called billboard creation, I need to create a material which based on the image file that imported by the user. I read the Revit API documents and figure out to create such a material need to follow these steps: 

1.Create a materail by document;

2.Create an ApperanceAssetElement by asset(the asset is find by Apllication.get_assets(AssetType.Appearance).Cast<Asset>.FirstOrDefault())

3.Assign the ApperanceAssetElement to the material.AppearanceElementId

But after that, I don't know how to set the Image path of the asset or the ApperanceAssetetElement.