ttkbootstrap buttone image not display

Question

I want to use ttkbootstrap to add an image to a button. However, the PNG image is not visible. What could be the problem with the code below?

Thanks a lot.

import ttkbootstrap as tkb
from pathlib import Path
from ttkbootstrap.constants import *

class test(tkb.Window):
    def __init__(self, title="ttkbootstrap", themename="litera", iconphoto='',\
                 size=None, position=None, minsize=None, maxsize=None, resizable=None, \
                 hdpi=True, scaling=None, transient=None, overrideredirect=False, alpha=1):
        super().__init__(title, themename, iconphoto, size, position, minsize, maxsize, resizable, hdpi, scaling, transient, overrideredirect, alpha)
        IMG_PATH = Path(__file__).parent / 'images'
        images = [
            tkb.PhotoImage(name='main', file=IMG_PATH/'icons8-settings-32.png')
        ]
        def image(): 
            tkb.Button(self, image=images[0]).pack(expand=1)

        image()

if __name__=='__main__':
    app = test()
    app.mainloop()

Not display PNG Images