The image loaded in Tkinter for button don't appear

Question

I'm creating an application to download YouTube videos without payment using Python, and everything is working well in terms of creating the screen. However, to make it cleaner and more beautiful, I want to add an image to the download button. As mentioned in the title, the image will be blank. here is the code:

from pytube import YouTube
import tkinter as tk
from tkinter import ttk

class Screen:
    def __init__(self, entry_widget, category_combobox):
        self.entry = entry_widget
        self.category_combobox = category_combobox
        self.screen_instance = (self.entry, self.category_combobox)
        
        
        
    def ErrorMessage(self,Message):
        ErrorWd = ttk.Frame(master=window)
        
        
        
        

    # Make the change in parameter of Download for which the user chose
    def get_stream_by_category(self, selected_category, video): 
        category_to_itag = {
            'mp4': 137,
            'webm': 248,
            'AudioOnly': 251,
        }

        itag = category_to_itag.get(selected_category, 'default')
        if itag == 'default':
            return video.streams.get_highest_resolution()
        else:
            return video.streams.get_by_itag(itag)

    # Download the video, getting the URL and category selected in inputs by the user
    def DownloadVideo(self): 
        url = self.entry.get()
        selected_category = self.category_combobox.get()

        video = YouTube(
            url, 
            on_progress_callback=print("just there"),
            on_complete_callback=print("Success!"),
            proxies=None,
            use_oauth=False,
            allow_oauth_cache=True
        )

        stream = self.get_stream_by_category(selected_category, video)

        stream.download()
        self.entry.delete(0, 'end')

# This condition makes the variables in __init__ accessible for everyone
if __name__ == '__main__':
    # Create the main window and specify some characteristics
    window = tk.Tk()
    window.title('SpoTube')
    window.geometry('726x512')
    window['background'] = '#242424'

    # Make an instruction label for the user
    def UserInstructionConstruct(window):
        UserInstruction = ttk.Label(master=window, text='Insert a link to download!',
                                    font='poppins 15 bold', background='#242424', foreground='white')
        UserInstruction.pack(pady=20)

    UserInstructionConstruct(window)

    input_link = ttk.Frame(master=window)
    entry = ttk.Entry(master=input_link)

    categories = ["AudioOnly", "webm", "mp4"]

    # Create a Combobox for selecting a category
    category_var = tk.StringVar()
    category_combobox = ttk.Combobox(input_link, values=categories, textvariable=category_var, state="readonly", width=5)
    category_combobox.pack(side='bottom', pady=10)

    screen_instance = Screen(entry, category_combobox)

   

    def CreateDownloadButton():
         # Load rounded button image
        load_image = tk.PhotoImage(file="SpotTubeApp/BlackDownloadButton.png")
        button = ttk.Button(master=input_link,width=5,image=load_image, command=screen_instance.DownloadVideo, compound="center")
        button['style'] = 'RoundedButton.TButton'
        button.pack(side='bottom', padx=5, pady=10)

    CreateDownloadButton()

    entry.pack(side='left', padx=10)
    input_link.pack(pady=125)

    # Configure a custom style for the rounded button
    style = ttk.Style()
    style.configure('RoundedButton.TButton', padding=10, relief="flat")

    window.mainloop()

I tried change the image, change the paths, and try another images. I just want a way to make the buttons and inputs to look more like CSS flat design style. Sorry if my english or question isn't clear, I'm learning both, Lol.