多言語サイトで画像SEOを最適化する方法

Search engines reward fast pages. For most photographs and blog images, WebP (or AVIF where supported) beats heavy JPEG/PNG on bytes-per-quality. Keep PNG for UI, screenshots with text, and assets that need perfect alpha. Compare tradeoffs in WebP vs JPEG and WebP vs PNG.

Start with practical bands, then validate on real creatives:

• WebP photos: quality 78–86
• JPEG photos: quality 80–88
• AVIF: often 45–62
• Transparent logos: WebP/PNG near-lossless

Use the image compressor and resize tool before CMS upload so plugins do not guess quality for you.

Name files like blue-running-shoes-side.webp, not final-final2.webp. Alt text should describe the image in context of the page — helpful for accessibility and topical clarity. Surround images with relevant headings and captions; do not stuff keywords into every attribute.

• LCP: Preload or prioritize the hero image; serve a correctly sized modern format.
• CLS: Reserve space with width/height or CSS aspect-ratio.
• INP: Avoid huge decoded bitmaps that block the main thread on low-end phones.

Convert delivery assets with JPG to WebP or PNG to WebP, then re-test PageSpeed on mobile.

Each locale should link to same-language tools and guides. A Japanese article that points to English converters dilutes relevance and confuses users. Keep canonical + hreflang aligned, and localize filenames/alt text when the page language changes.