I had some trouble when loading image.
You may need to insert a delay before loading the texture:
var image = Image.new()
var error = image.load_jpg_from_buffer(img_buffer)
yield(get_tree(), "idle_frame") <--- IMPORTANT!
var texture = ImageTexture.new()
texture.create_from_image(image, 0)
get_node("TextureRect").texture = texture