What is JPEG?
JPEG (Joint Photographic Experts Group) was standardized in 1992 and quickly became the most widely used image format for photographs. It uses lossy compression based on the Discrete Cosine Transform (DCT), which converts spatial pixel data into frequency components. The human visual system is less sensitive to high-frequency detail, so JPEG discards some of this information to achieve dramatic size reductions. JPEG supports 24-bit color (16.7 million colors) and stores EXIF metadata including camera settings, GPS coordinates, and timestamps.
How JPEG Compression Works
JPEG compression involves four main steps: color space conversion (RGB to YCbCr, where chroma channels can be subsampled), block splitting (the image is divided into 8x8 pixel blocks), DCT transformation (each block is converted to frequency coefficients), and quantization (frequency coefficients are divided by values from a quantization table, rounding to zero the less important high-frequency data). The quality slider controls the quantization table — lower quality means more aggressive rounding, smaller files, but more visible artifacts like blocking and ringing around edges.
When to Compress JPEG Files
JPEG compression is ideal for photographs, natural scenes, portraits, and any image with smooth color gradients. Use it for website hero images, product photography, blog post illustrations, social media uploads, and email attachments. JPEG is not recommended for images with sharp text, line drawings, or graphics requiring transparency — use PNG or WebP for those. Compress JPEGs whenever you need to reduce page weight for faster Core Web Vitals scores or fit within email size limits.
JPEG Compression Best Practices
Use quality 75-85 for web images — this range provides excellent visual quality with significant size savings. Always resize images to their display dimensions before compressing. Strip EXIF metadata to save 10-50 KB per image (unless you need camera info). Enable progressive JPEG encoding for better perceived loading on slow connections. For critical above-the-fold images, test quality settings visually rather than relying on a fixed number. Consider switching to WebP for 25-34% better compression at equivalent quality.
