Converter Imagens (usando Tkinter)

from tkinter import *
from tkinter import filedialog
from PIL import Image
root =Tk()
class imagens():
    def __init__(self):
        self.root = root
        self.janela()
        self.frac()
        root.mainloop()
    def janela(self):
        self.root.title("Converter Imagem")
        self.root.geometry("450x200")
        self.root.resizable(False, False)
    def frac(self):
        self.texto1_lb = Label(text="Converter Imagem",
                                  font=("Helvetica", '15'))
        self.texto1_lb.place(relx=0.3, rely=0.05)
        # Butões
        #
        self.bt_png = Button(text="Selecionar um ficheiro PNG ", bd=2,
                                 font=('verdana', '8', 'bold'), bg='gray18',
                                  fg='white',command=self.selecionarimagem)
        self.bt_png.place(relx=0.25, rely=0.3, relwidth=0.5, relheight=0.15)

        self.bt_converter_png = Button(text="Converter PNG para JPG ", bd=2,
                             font=('verdana', '8', 'bold'), bg='gray18',
                             fg='white',command=self.converterimagem)
        self.bt_converter_png.place(relx=0.25, rely=0.6, relwidth=0.5, relheight=0.15)

    def selecionarimagem(self):
            global im1
            import_file_path = filedialog.askopenfilename()
            im1 = Image.open(import_file_path)

    def converterimagem(self):
        global im1
        export_file_path = filedialog.asksaveasfilename(defaultextension='.jpg')
        im1.save(export_file_path)
imagens()