A digital image is a grid of pixels. Each pixel stores a colour as a binary number.

- Resolution — width × height in pixels. A 1920 × 1080 photo is 'Full HD'.

- Colour depth — bits per pixel. 1-bit = 2 colours (black/white). 24-bit = 16.7 million colours (true colour).

Both affect file size and quality: more pixels and more colour depth = sharper image, bigger file.