Create placeholder images at the aspect ratios and dimensions the finished layout will use. A square for an avatar and a wide image for a card reveal different cropping and spacing problems.
Label each placeholder with its intended role or dimensions so it cannot be mistaken for finished art. Preview the page at phone and desktop widths; ensure the layout remains stable when real images load. Replace placeholders before publishing and test with a realistic light and dark image, not only a neutral block.
The purpose of a placeholder is to expose layout assumptions early.
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