Start from the placement
Check the largest rendered size and whether the layout uses multiple responsive sizes. A file that is tiny on desktop may be unnecessarily large for a small card.
Consider high-density screens, but do not upload an oversized original to every context without checking the site's delivery workflow.
Choose format and dimensions together
Resize dimensions before comparing compression settings. Use a format supported by the destination and preserve transparency when it matters.
Inspect the actual output. A modern format can reduce size, but content, encoder settings, and browser support affect the result.
Give the image context
Write useful alternative text when an image conveys information or performs a function. Decorative images should not add redundant announcements.
Do not encode instructions, prices, or essential text only inside an image when the page can provide that information as text.
Common questions
Can compression improve page accessibility?
Not by itself. File size can affect loading, but useful alt text, keyboard access, contrast, and page structure are separate requirements.
Should all images use the same dimensions?
No. Use dimensions suited to the content and layout. Cropping a portrait, logo, diagram, and landscape photo to one shape can remove useful information.
References
Sources explain browser behavior or guidance; they do not verify a particular output file.