How can I preserve PNG transparency when displaying an image in Tkinter?

Your code actually looks like it should work as written. However, I'd generally recommend using Pillow for this kind of situation. Sorry, I know "use a different library" isn't what anyone wants to hear! The main reason is that Pillow is specifically designed for loading and processing images, while Tkinter has this built-in image support that is relatively base-level.

If you do end up using PIL.Image, ImageTk.PhotoImage makes it fairly easy to convert a Pillow image into a format that Tkinter knows how to display, while still allowing you to use all the tools provided by Pillow.

If I were you, I would use Pillow for handling images in the first place, however if you merely want to use tkinter without any other hassles, you can just render it on a canvas instead of in a label. Try something like this:

import tkinter as tk
root = tk.Tk()
root.title("Native Tkinter Transparency")
root.geometry("500x400")
root.configure(bg="#2c3e50")
canvas = tk.Canvas(root, width=400, height=300, bg="#2c3e50", highlightthickness=0)
canvas.pack(pady=20)
transparentimg = tk.PhotoImage(file="logo.png")
canvas.createimage(200, 150, image=transparentimg)
canvas.image = transparentimg
root.mainloop()

Pillow generally works better and more consistently merely because Tkinter is an old library and lacks image processing abilities. In fact, I would recommend moving away from the standard tkinter library in totality. If you want an easy work around for everything (rounded corners etc,..), use CustomTkinter, which doesn't require any of the extra jazz!

Good luck.