python tkinter canvas 显示图片

先来看一下该方法的说明

create_image(position, **options) [#]
Draws an image on the canvas.

position
Image position, given as two coordinates.
**options
Image options.
activeimage=
anchor=
Where to place the image relative to the given position. Default is CENTER.
disabledimage=
image=
The image object. This should be a PhotoImage or BitmapImage, or a compatible object (such as the PIL PhotoImage). The application must keep a reference to the image object.
state=
Item state. One of NORMAL, DISABLED, or HIDDEN.
tags=
A tag to attach to this item, or a tuple containing multiple tags.
Returns:
The item id.

关于image有两个重要的点要注意，一个是格式，第二是要保持持续引用
The image object. This should be a

1.This should be a PhotoImage or BitmapImage, or a compatible object (such as the PIL PhotoImage).
2.The application must keep a reference to the image object.

因此代码应该这样写，并且变量im应该是全局变量

image = Image.open("img.jpg")  
im = ImageTk.PhotoImage(image)  

canvas.create_image(300,50,image = im)  

但如果我就是想要在方法里调用怎么办？
那么可以提前声明全局变量

image = None
im = None

之后在方法里使用global来声明变量为全局变量
即：

def method():
    global image
    global im
    image = Image.open("img.jpg")  
    im = ImageTk.PhotoImage(image)  
    ...