JPEG
Dominant lossy image compression. ISO standard 1992. Discrete Cosine Transform-based. 30+ years old, still nearly default for photography on the web.
Compression: block-based DCT + quantization + Huffman coding. Quality factor 1-100 controls size vs quality. Artifacts at high compression: blocking, ringing. Successors: JPEG 2000 (wavelet, never popular), WebP (Google, 2010), AVIF (Netflix/AOM 2019, dominates modern web), JPEG XL (2022, theoretically better than AVIF but lost browser support — Chrome dropped it). HEIC is iPhone default.