Use SVGs (scalable vector graphics) where possible because they look the best at any size.
Vectors use maths to ensure they always look sharper than James Bond on a night out.
You can open SVGs in a web browser, they’re especially useful in MS Word & PowerPoint.
We can use PNGs as a backup as they are widely accepted and have a transparent background but be careful if you re-size or stretch a PNG as they will pixelate.
Never use JPG unless you absolutely have to, otherwise you are forced to have a white background and they pixelate badly. JPG is a format intended for photographs, not vectors.