viewer.py

from Tkinter import *
import glob

class MyWindow(object):
    def __init__(self):
        self.label = None
        self.root = Tk()
        self.i = 0
        self.files = glob.glob('./*.png') + glob.glob('./*.jpg') + glob.glob('./*.jpeg')

        self.left = Button(self.root, text="Left", command=self.lclick)
        self.right = Button(self.root, text="Right", command=self.rclick)

        self.frame = Frame(self.root, width=500, height=500)

        self.left.pack()
        self.right.pack()
        self.frame.pack()

        self.update()

        self.root.mainloop()

    def lclick(self):
        self.i = self.i + 1
        if self.i >= len(self.files):
            self.i = 0
        self.update()

    def rclick(self):
        self.i = self.i - 1
        if self.i <= 0:
            self.i = len(self.files) - 1
        self.update()

    def update(self):
        if self.label:
            self.label.destroy()

        self.photo = PhotoImage(file=self.files[self.i])
        self.label = Label(self.frame, image=self.photo, width=500, height=500)
        self.label.pack();

m = MyWindow()