Selecting the right image format requires balancing file size, visual quality, and feature requirements like transparency. For decades, the web relied on a strict dichotomy: JPEG for complex photographs, and PNG for graphics requiring transparency or sharp lines.
JPEG uses lossy compression, specifically discrete cosine transform (DCT), which discards high-frequency visual data the human eye struggles to see. This results in incredibly small files for photos, but creates ugly artifacts (ringing) around sharp text and flat graphics.
The role of PNG and lossless compression
PNG (Portable Network Graphics) is fundamentally different. It uses the lossless DEFLATE algorithm (the same math behind ZIP files). When you save a PNG, no original pixel data is thrown away, and it perfectly preserves sharp edges.
Crucially, PNG supports an alpha channel for true transparency. The downside is file size. Saving a complex 12-megapixel photograph as a PNG results in a massive 15-30 MB file, because the lossless algorithm struggles to compress the millions of subtle color variations found in natural photography.
The modern era: WebP and AVIF
WebP, developed by Google, aimed to replace both formats. It supports both lossy and lossless modes, and it includes alpha transparency in both. A lossy WebP can be 25-34% smaller than an equivalent quality JPEG, while also allowing for transparent backgrounds.
AVIF is the newest standard, derived from the AV1 video codec. It routinely outperforms WebP, offering up to 50% smaller file sizes than JPEG at the same visual quality. Furthermore, AVIF supports 10-bit and 12-bit color depth and HDR, making it the definitive format for modern, high-performance web development, assuming browser support allows it.