GOV.UK is unusually specific about digital passport photos, which makes the job straightforward. The photo must be clear and in focus, in colour, unaltered by computer software, at least 600 pixels wide and 750 pixels tall, and at least 50KB but no more than 10MB. It must be taken in the last month, against a plain light-coloured background, with no other objects or people in frame. Then the instruction that catches people out: if you are using a photo from your own device, include your head, shoulders and upper body, and do not crop the photo — it will be done for you. So the correct workflow is to take a wide, well-lit shot and leave the framing alone. Compression is then purely a size question, and the range is wide: 50KB at the floor, 10MB at the ceiling. A photo booth or shop code is still the most likely to be approved, but a homemade photo that respects those bounds is accepted frequently. The failure modes are a file under 50KB that looks mushy, an over-sharpened or filtered image, and a crop that cut the shoulders out. Skwish resizes to exact pixel dimensions and compresses to a KB target in the browser, shows the before and after, and runs entirely on your device.