What you need to know
A camera photo or export may be many times larger than the displayed area needs. Smaller responsive files reduce download time and can lower data usage for mobile visitors.
How to apply it
Resize close to the maximum rendered dimensions, use modern formats where supported, set quality by visual inspection, and provide variants for different screen widths.
Common mistake to avoid
Do not upload the original multi-megabyte image and expect CSS to make the download small. CSS changes display size, not the source bytes transferred.