What is How to compress a JPEG without losing quality?
TL;DR: To compress a JPEG without visible loss: keep quality at 78-85%, cap the resolution to what you actually need, and re-encode once. Shrink44 does both in your browser.
JPEG is already a compressed format, but photos straight from a phone or camera are saved at near-maximum quality and full resolution — far more than most uses need.
The two levers that matter: quality (78-85% is visually identical to the original) and resolution (cap the longest side to your real display size). Together they cut 60-80% with no visible difference.