Sepia gives an image a warm tonal treatment. It is useful as a visual style, but it does not selectively replace the colours of particular objects or simulate a specific historical photographic process.
Sepia Image applies a defined RGB colour transform to the whole image. The dimensions remain the same.
See the transform in a simple colour
For an opaque red pixel with RGB 255, 0, 0, the transform produces approximately RGB 100, 88, 69 after integer conversion. That warm mixture illustrates why sepia is more than putting a flat brown layer over the image.
Bright combinations can reach the maximum channel value and clip. The result therefore needs review in highlights as well as midtones.
Existing alpha is retained by the pixel transform. PNG is suitable when those transparent areas must remain reusable over different backgrounds.
Style can remove distinctions you need
A coloured badge, chart or product variant may become less distinguishable after the treatment. Do not use a sepia copy where accurate colour is essential to the task.
The operation does not improve resolution, repair scratches or remove noise. A poor source can still look poor after a warmer colour treatment.
Return to the original for a fresh comparison instead of applying sepia repeatedly to previous downloads. Each pass changes the already transformed values again.
Inspect the file at the size and background where it will appear. A warm effect that looks attractive as a large preview may be too dark or muddy in a small thumbnail. Keep the copy that serves the intended presentation rather than treating the effect as an automatic improvement.