The following script creates a transparent GIF with a red circle drawn in the middle:
from PIL import Image, ImageDraw
img = Image.new('RGBA', (100, 100), (255, 0, 0, 0))
draw = ImageDraw.Draw(img)
draw.ellipse((25, 25, 75, 75), fill=(255, 0, 0))
img.save('test.gif', 'GIF', transparency=0)
and for PNG format:
img.save('test.png', 'PNG')