The main image manager in PIL is PIL's Image module.
from PIL import Image
import math
foo = Image.open("path\\to\\image.jpg")
x, y = foo.size
x2, y2 = math.floor(x-50), math.floor(y-20)
foo = foo.resize((x2,y2),Image.ANTIALIAS)
foo.save("path\\to\\save\\image_scaled.jpg",quality=95)
You can add optimize=True to the arguments of you want to decrease the size even more, but optimize only works for JPEG's and PNG's.
For other image extensions, you could decrease the quality of the new saved image.
You could change the size of the new image by just deleting a bit of code and defining the image size and you can only figure out how to do this if you look at the code carefully.
I defined this size:
x, y = foo.size
x2, y2 = math.floor(x-50), math.floor(y-20)
just to show you what is (almost) normally done with horizontal images. For vertical images you might do:
x, y = foo.size
x2, y2 = math.floor(x-20), math.floor(y-50)
. Remember, you can still delete that bit of code and define a new size.