Ten kilobytes is a very small file, and the limit usually comes from an online application form, where a scanned signature or a thumbnail photo has to fit under it. A phone photo is often 3 to 5 MB, so getting it under 10 KB means reducing it by around 99.7 percent, and quality alone cannot do that. This page opens with the target set to 10 KB. The tool lowers the JPG quality first and, when that is not enough, scales the image down step by step until it fits, keeping the aspect ratio. The result for a photo is usually around 150 to 250 pixels on the long side, which is enough for an ID style thumbnail but not for a printable image. Signatures do much better, because a dark line on a white background compresses extremely well. For the cleanest signature, crop tightly around it before compressing so the pixels go to the ink, not the empty paper, and scan or photograph it in good light so the background is white rather than grey. The target is counted as 10,000 bytes, so the file is under the limit whether the form counts a kilobyte as 1,000 or 1,024 bytes. Crop first with Image Cropper, and for a slightly larger limit see Compress Image to 20KB.
Image compression is one of the highest-impact performance optimizations for most websites. Images typically account for 50 to 80 percent of a page's total download size, and unoptimized images are the most common cause of poor Core Web Vitals scores, particularly Largest Contentful Paint (LCP). A JPEG compressed to 75 percent quality is visually indistinguishable from the original for photographic content in most contexts, but can be 60 to 70 percent smaller. The human eye is much more sensitive to sharpness and color accuracy in certain areas (faces, text in images, sharp edges) than in others (smooth backgrounds, sky, blurred backgrounds). Lossy compression algorithms exploit this by applying heavier compression in less sensitive areas. For PNG files, lossless compression reorders pixel data and uses more efficient encoding tables. PNG compression level (1-9) trades CPU time for file size: higher levels take longer to encode but produce smaller files. For web use, both JPEG and PNG should be served at their maximum practical compression level (JPEG 75-85, PNG level 9) since the decompression time is negligible but the smaller network transfer size benefits every user. The recommended modern workflow for web image delivery is: optimize your original image with this tool, then serve it in WebP format for Chrome and Safari 14+ users with a JPEG fallback for older browsers using the HTML picture element with source type="image/webp".
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