Displays images from a folder on a rotation basis

Picture Rotation.py

#!/usr/bin/env python
import PySimpleGUI as sg
import os
from sys import exit as exit
from  PIL import Image
import io

# Helper func
def image_file_to_bytes(filename, size):
    try:
        image = Image.open(filename)
        image.thumbnail(size, Image.ANTIALIAS)
        bio = io.BytesIO()  # a binary memory resident stream
        image.save(bio, format='PNG')  # save image as png to it
        imgbytes = bio.getvalue()
    except:
        imgbytes = None
    return imgbytes

# Get the folder containing the images from the user
folder = sg.PopupGetFolder('Image folder to open')
if folder is None:
    sg.PopupCancel('Cancelling')
    exit(0)

# get list of PNG files in folder
png_files = [os.path.join(folder, f) for f in os.listdir(folder) if '.png' in f]
filenames_only = [f for f in os.listdir(folder) if '.png' in f]

if len(png_files) == 0:
    sg.Popup('No PNG images in folder')
    exit(0)
# -------------------- BEGIN GUI SECTION --------------------
layout = [[sg.Graph((500,500), (0,0), (500,500), background_color='black', key='_GRAPH_', enable_events=True)],
          [sg.B('Next'), sg.B('Exit')]]

window = sg.Window('Image Browser',
                   layout,
                   no_titlebar=True,
                   grab_anywhere=True,
                   background_color='black',
                   use_default_focus=False )

# -------========= Event Loop =========--------
display_index=0
prev_name = None
while True:
    event, values = window.Read(timeout=1000)
    if event in (None, 'Exit'): break
    cur_filename = png_files[display_index]
    next_index = display_index + 1 if display_index+1 < len(png_files) else 0
    next_name = png_files[next_index]
    if prev_name:
        imgbytes = image_file_to_bytes(filename=prev_name, size=(200,200))   # draw previous file
        window.Element('_GRAPH_').DrawImage(data=imgbytes, location=(325,300))
    if next_name:
        imgbytes = image_file_to_bytes(filename=next_name, size=(225,200))   # draw previous file
        window.Element('_GRAPH_').DrawImage(data=imgbytes, location=(0,300))
    imgbytes = image_file_to_bytes(filename=cur_filename, size=(500,500))  # draw current filename
    window.Element('_GRAPH_').DrawImage(data=imgbytes, location=(125,400))
    display_index = display_index + 1 if display_index+1 < len(png_files) else 0
    prev_name = cur_filename
window.Close()