How can I find the resolution of an image with any extension without using any module that is not part of Python's standard library (except perhaps PIL)?
You can get an image's resolution with the Pillow package:
from PIL import Image
with Image.open("filename") as image:
width, height = image.size
You can find more in the documentation
As pippo1980 pointed out, this is for the dimensions of the image and not for the image's resolution. For completeness, here's how to get an image's resolution:
from PIL import Image
with Image.open("filename") as image:
xres, yres = image.info['dpi']
If the info dictionary doesn't have a dpi key, it's likely because the image is missing metadata required to calculate that (pixels per inch, or something similar relating unit/distance)