Choose the right format first
JPEG works well for photos and complex scenes. PNG is useful for sharp graphics, transparency, and screenshots with text. WebP can be efficient for many web uses.
Changing format can matter more than changing quality settings. Test the format that matches the image type.
Inspect details after compression
Look at faces, product labels, small text, gradients, and shadows. These areas show compression problems first.
Do not judge only by file size. A tiny file that makes text unreadable is not a successful optimization.
Keep an original copy
Compression is usually lossy for photos. Save a separate optimized copy and keep the original in a safe folder.
If you need multiple sizes, create each size from the original rather than repeatedly compressing an already compressed file.