Error when trying to open new window with label+image in tkinter

Question

I ran into a probleme when coding a bigger project (at least for me as an absolute newby). I want to open a new tk-window with some text and an image by pressing a Button from another tk-window. I created a code for demonstration.

import tkinter as tk

def fct():
    testwin = tk.Tk()

    testbutt1 = tk.Button(testwin, text="Button1").pack()

    img = tk.PhotoImage(master=testwin,file="picture.png")
    imglabel = tk.Label(root,image=img)
    imglabel.pack()

    testbutt2 = tk.Button(testwin, text="Button2").pack()

    testwin.mainloop()

root = tk.Tk()
picbutt = tk.Button(root,text="Exit",command=fct).pack()

label = tk.Label(root)
label.after(5000, fct)

root.mainloop()

This code throws error: "_tkinter.TclError: image "pyimage1" doesn't exist"

When I exchange img = tk.PhotoImage(master=testwin,file="picture.png") to img = tk.PhotoImage(testwin,file="picture.png") I get error: "images may not be named the same as the main window"

Changing import tkinter as tk to from tkinter import * sadly doesn't change anything either. It also doesnt matter if fct() is called by pressing the button or waiting 5 seconds (label.after(5000, fct))

Python stops executing code at this point. Meaning, testbutt1 shows up, testbutt2 does not.

Hope this explaination is enough to describe the probleme. Thanks for your answers :)