ui_tex = bpy.data.textures.new('MP', 'IMAGE')

ui_tex.image = bpy.data.images.load("D:/pic.png")

class UIPanel(bpy.types.Panel):

  bl_label = "Label"
  bl_space_type = "VIEW_3D"
  bl_region_type = "TOOLS"
  br_name = "Printing"

  def draw(self, context):

   layout = self.layout
   scn = context.scene

   ui_tex = bpy.data.textures.new("MP", "IMAGE")
   ui_tex.image = bpy.data.images.load("D:/pic.png")

   row = layout.row(align=True)
   row.alignment = 'EXPAND'
   row.template_preview(bpy.data.textures["MP"])

line 39, in draw
ui_tex.image = bpy.data.images.load(filepath = filepath)
AttributeError: Writing to ID classes in this context is not allowed: MP, Texture datablock, error setting ImageTexture.image

bpy.ops.image.open(filepath='full_path_to/your_image.png')
bpy.data.images['your_image.png'].pack(as_png=True) # Pack an image as embedded data into the .blend file
ui_tex.image = bpy.data.images['your_image.png']