How to resize an image without losing quality, and when you cannot

'Without losing quality' means two different things depending on direction. Making a photo smaller throws away pixels you would not have seen anyway. Making it bigger asks the computer to invent pixels that were never captured.

Smaller: safe, if done once

Shrinking a 4000-pixel photo to 1200 pixels averages groups of pixels into one. Done with good resampling, as the browser does here, the result looks sharp at its new size. The loss to avoid is resizing several times in a row, or resizing an already heavily compressed JPG; each JPG save adds a little blur. Start from the original and resize once.

Bigger: no tool adds real detail

Enlarging spreads the same information over more pixels, so edges get soft. Doubling the size is usually acceptable for a screen; four times looks visibly blurry. The resize tool's 'Never enlarge' option keeps small images at their size instead of upscaling them. If a portal demands more pixels than you have, retake or rescan the photo at a higher resolution rather than enlarging.

Changing the shape without stretching

When the target has a different aspect ratio, say a 3:4 phone photo into a 7:9 ID frame, there are three choices. Fit inside keeps the whole picture and leaves the box partly empty or smaller; crop to fill trims the edges around the centre so the frame is full; stretch fills the frame by distorting the picture, which faces and text make obvious. Crop to fill is right for photos of people; use the crop tool first if the subject is off-centre.

The order that keeps quality

• Start from the original file, not a copy from a chat app.
• Crop to the subject.
• Resize once to the size the destination displays or requires.
• Save at quality 85 to 90, or let the fit-to-size tool choose the highest quality that meets a KB limit.

Pixels, DPI and print size

DPI does not change the pixels; it only tells a printer how large to print them. A 600 × 600 photo is 2 × 2 inches at 300 DPI and 4 × 4 inches at 150 DPI. When a size is given in centimetres or inches, the resize tool converts it to pixels at the DPI you choose and writes that DPI into the JPG or PNG, so it prints at the intended size.