Choosing an image format is one of the most subtle yet critical tasks for a website. Even if the design looks perfect, large files increase page load times, degrade the user experience, and can negatively impact search rankings. Below are practical recommendations on which formats are generally best and why size and weight are important. 1) The best formats for a website: WebP and AVIF WebP is one of the most popular options: it's supported by most modern browsers and typically offers good compression without noticeable quality loss. AVIF often provides even more efficient compression (smaller file size with comparable quality), but compatibility is important: current browser versions support it better than before. If your goal is optimal performance, the most common strategy is to serve AVIF where it is supported, and WebP as a reliable fallback. 2) JPEG, PNG, and GIF – when appropriate JPEG is generally suitable for photographs and images with complex gradients: the format compresses data well, especially at reasonable quality settings. For halftones and "photo-like images," it's often more compact than PNG. PNG is useful when transparency or clear elements with sharp edges are needed (for example, logos with transparent backgrounds). However, PNG can be heavier, so it's best to use it sparingly. GIFs are used much less frequently today. If animation is important, it's often better to consider modern alternatives (such as video or formats with better compression), as GIFs typically achieve a larger file size for the same visual purpose. 3) Why image size matters "Size" refers not only to the width and height in pixels, but also to the actual file size. The larger the file size, the longer it takes the browser to download the image, the higher the network load, and the longer the page remains "incompletely loaded." This is especially noticeable on mobile connections. In addition to loading speed, page size affects how pages are indexed and perceived by users. The faster a site responds, the more likely a visitor is to view the page to the end and complete the desired action. A separate important point is matching the actual dimensions to the page display. If a 4000x3000 pixel image is used in a block sized 800x600, it will still be heavy, even if it "visually" appears small. Proper scaling and resizing to the actual layout dimensions can significantly reduce the weight. Bottom line: choose a format with the best compression (usually WebP/AVIF), reduce unnecessary pixels, and don't forget to optimize quality. Simple steps often yield significant gains: exporting to the correct format, selecting the quality level for JPEG/WebP, and converting to PNG if transparency isn't needed. 4) Recommended approach for different types of content Below are guidelines that typically work well on sites with different content. Photos: JPEG or WebP; supported by AVIF. Images with transparency: PNG (or WebP/AVIF with alpha channel, if supported by your stack). Icons and logos: PNG for transparency or vector (if possible), otherwise optimized PNG/WebP. Animations: Avoid GIFs by default; choose a more efficient format/approach to the task. Background graphics and banners: consider the weight and actual display dimensions on the site. To ensure that your solutions are actually improving your metrics, it's helpful to check your loading times in performance tools to see how much each image type weighs and how quickly it appears on the page. Therefore, the best format is the one that delivers the desired quality with minimal file size and good browser support. And size (both pixel and file size) is important because it directly determines loading speed, user experience, and SEO effectiveness.