To load a texture from a file located inside your project, this is the preferred way:
texture = load("res://path/to/your/texture.png")
To load a texture from a file located anywhere on the filesystem, provided your game has the access rights:
var image = Image.new()
var err = image.load("path/to/the/image.png")
if err != OK:
texture = ImageTexture.new()
Note that this second way will not have gone through import, so any extra options need to be specified manually. It should be preferred for savegames or images provided by the player.