public enum esriTextureCompressionType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable  

/// <summary>
/// This method gets the material texture image of a multipatch.
/// This method must be called on the MCT. Use QueuedTask.Run.
/// </summary>
/// <param name="multipatch">The input multipatch.</param>
/// <param name="patchIndex">The index of the patch (part) for which to get the material texture.</param>
public void GetMultipatchTextureImage(Multipatch multipatch, int patchIndex)
{
  int materialIndex = multipatch.GetPatchMaterialIndex(patchIndex);
  if (!multipatch.IsMaterialTextured(materialIndex))
    return;

  esriTextureCompressionType compressionType = multipatch.GetMaterialTextureCompressionType(materialIndex);
  string ext = compressionType == esriTextureCompressionType.CompressionJPEG ? ".jpg" : ".dat";
  byte[] textureBuffer = multipatch.GetMaterialTexture(materialIndex);

  Stream imageStream = new MemoryStream(textureBuffer);
  System.Drawing.Image image = System.Drawing.Image.FromStream(imageStream);
  image.Save(@"C:\temp\myImage" + ext);
}