Start with the right dimensions
A very large image often contains more pixels than the final use needs. Resize it to the approximate display dimensions before increasing compression. This can reduce file size substantially without forcing a low-quality setting.
Choose a suitable format
JPEG is often efficient for photographs. PNG is useful when transparency or crisp flat graphics matter. WebP can provide a good balance for many web images.
Compress gradually
Make one change at a time and compare the result with the original. Look closely at faces, small text, gradients and sharp edges.
For a strict upload limit
If a website requires a maximum file size, first resize unnecessary dimensions, then adjust compression toward the target.