Python Tkinter button to advance through a folder of PNG files

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Python

GUI

PNG

So I don't even know where to begin writing this part of my code --> so there is little sample code to post.

I have figured out how to create PNG files using matplotlib and I want to display them one at a time and advance through them at the click of a TKinter button.

def nextDay():
    print("Next Day")
root = Tk()
photo = PhotoImage(file = "example69.png")
label = Label(root, image=photo).grid(row=0, rowspan=25, column=0)
button4 =Button(root, text="Next Day", command=nextDay).grid(row=3, column=1, sticky=E+W)
root.mainloop()

I can load and display the first one, but I'm really staring at the void when it comes to defining a button function (nextDay) that loops through and changes the photo variable to the next PNG. Do I only need to change it once and let the main loop refresh it? Or do I need to keep a list of PNG files ready to loop through? I have no idea on the latter.

Any suggestions would be greatly appreciated.