JPG / JPEG

JPG (also written JPEG, after the Joint Photographic Experts Group that defined it) is the lossy raster format built for photographs. It works by transforming the image into frequency components and discarding the fine detail the human eye is least likely to miss, which is how it squeezes a full-colour photo into a small file.

The central control is the quality setting. Higher quality keeps more detail and produces a larger file; lower quality saves space but introduces visible artefacts, the blocky patches and haloing around sharp edges that appear when compression is pushed too far. JPEG is lossy and generational: every time you re-save a JPEG it is recompressed and loses a little more, so repeated edits degrade it. It also has no transparency, which is why it suits photos but not logos or screenshots.

Inside PDFs, photographic images are very often stored as JPEG streams, and choosing a sensible quality is the single biggest lever on a document's file size. When you turn JPGs into a PDF or pull images out of one, doing it in the browser means the photos never leave your device.