How to reduce image file size
Three things shrink an image, in descending order of effect: convert it to WebP, reduce its pixel dimensions to what is actually displayed, and lower the quality setting. The first alone usually removes 70–90% of the bytes of a photographic PNG, which is more than the other two combined.
Start with format, not quality
People reach for the quality slider first and it is the weakest of the three levers. A photograph stored as PNG is carrying enormous overhead simply because PNG refuses to discard anything; moving it to WebP at 82% typically cuts it by an order of magnitude with no visible difference.
If the image is flat-colour artwork rather than a photograph, the opposite is true: PNG is already efficient for it, and a lossy format will add smearing around the edges while saving less than you expect.
Then dimensions
File size scales with area. Halving the width quarters the pixel count, and roughly quarters the bytes. This is usually the second-largest win and costs nothing visually, because most images on the web are far larger than the space they occupy.
Quality last, and how far to push it
For WebP and JPG, 80–85 is the usual sweet spot: below about 70 you start seeing blocking in flat areas like skies and ringing around text. Judge it at 100% zoom on the actual image, not on a thumbnail, because artefacts hide at small sizes.
Never re-compress an already-compressed file repeatedly. Each save compounds the loss, and it is not recoverable. Keep a lossless master and export from it.