The short answer
Prepare images for their actual display size and inspect the encoded result. Format, quality, transparency and source dimensions affect the trade-off; converting a file does not guarantee a smaller output. Keep the original available so a lossy conversion is reversible at the asset level.
A worked example
A fictional article uses a small transparent diagram and a large photograph. PNG may preserve the diagram cleanly, while a WebP copy of the photograph can be compared at several quality settings. Re-encoding a compressed source as PNG may increase its size, so the editor compares actual bytes and visual detail rather than relying on a format slogan.
A practical checklist
- Record the source dimensions and the intended rendered width. Resize a copy when a very large source adds no useful detail to the page.
- Choose a supported output format and quality setting. Preserve transparency when needed, and inspect edges, labels and gradients in the exported file.
- Check output dimensions, file size and contextual alt treatment. Use responsive image markup where appropriate, and reserve layout space to reduce unexpected movement.
What to avoid
Avoid promising a universal compression percentage or assuming higher quality creates a smaller file. Browser conversion can remove metadata and change color; review the export before replacing a production asset. Animated input may become a still image in a canvas-based workflow.
A useful follow-up
Is converting JPG to PNG a compression strategy?
Not necessarily. PNG output can be larger; compare the actual result and choose the format for the content's needs.
