在 Tkinter 中将其用作背景时不显示图像

Question

Hy! I am trying to use an image as a background in a GUI created in Tkinter. It works fine with the same code on a different frame in this project, but it doesn't want to work in a different frame. I get no error, the frame is just blank. Thank you for your help!

Here I can use the picture as a background just fine:

def main_screen():
global screen
screen = Tk()
screen.geometry("600x750")
screen.configure(background="#022140")
screen.title("Hermes")
filename = PhotoImage(file="background.png")
filename_small = filename.subsample(2, 2)
background_label = Label(image=filename_small)
background_label.place(x=1, y=1, relwidth=1, relheight=1)
login_button = Button(text="Login", bg="#022140", height="2", width="30", command=login,
                      highlightbackground='#494B68')
screen.mainloop()

But here it won't work:

def login_sucess():
    global screen3
    screen3 = Toplevel(screen)
    screen3.geometry("500x400")
    filename = PhotoImage(file="main_theme.png")
    filename_small = filename.subsample(2, 2)
    background_label = Label(screen3, image=filename_small)
    background_label.place()

Thank you for your help!

Answer 1

It should be quite simple. When you're working with images in tkinter you always need to set a reference to that image on the widget you used it on. In other words do something like this:

from tkinter import Tk,Label,PhotoImage

root = Tk()
img = PhotoImage(file='background.png')
small_img=img.subsample(2, 2)
background_label = Label(root, image=small_img)
background_label.img=img
background_label.place(x=0, y=0, relheight=1, relwidth=1)
root.mainloop()

Especially when you create the Label holding the image in a functions scope or something similar it's important to set that reference since the variable to that Labels instance will be gone after the function call ends.

PS: To be more clear about master widgets and for small performance improvements always give a widget a master widget when initializing it. I know tkinter also automatically assigns the last created Tk instance as master itself but a) does that need unneccesary computing power and b) is it easier to keep track of what widget is a child of another and so on. ;)