filenames = [
    'webpage.html', 'document.odt', 'song.wav',
    'picture.jpeg', 'picture2.JPG',
    'icon.gif', 'logo.png',
]

# First, initialize an empty list
images = []

# Add only the interesting filenames
for filename in filenames:
    # Get the extension
    extension = filename.split('.')[-1].lower()
    # Append it if it is an image
    if extension in ('png', 'jpg', 'jpeg'):
        images.append(filename)

# Done!