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

images = [
    filename
    for filename in filenames
    if filename.split('.')[-1].lower() in extensions
]

# 'images' contains 'picture.jpeg', 'picture2.JPG',
# 'icon.gif', 'logo.png'