A dark picture may need a small brightness correction before it is shared. Raising brightness makes pixel values lighter, but it cannot recreate detail that the camera never captured.
Image Brightness applies a bounded brightness adjustment to the decoded image. Positive values lighten the result and negative values darken it.
Begin with a modest change
On an opaque red pixel with RGB 255, 0, 0, an increase of 20 leaves red at its maximum while the other channels become 20. A channel cannot rise beyond 255, so already bright areas can lose visible separation as they clip.
That explains why an image can become washed out when an adjustment is pushed too far. Light areas reach the ceiling while darker areas continue moving upward.
Use a small value first and inspect the darkest important region alongside a bright region. Improving one without damaging the other is the practical aim.
Brightness is not exposure recovery
A severely underexposed face may still have noise and missing texture after lightening. A blown white area will not gain its original detail by darkening it.
The adjustment is a general pixel operation, not a camera RAW exposure control or an automatic subject aware enhancement. For a photograph with a strong backlight, one global setting may not suit both the face and background.
Choose an output format that retains any needed transparency. Avoid converting to a low quality JPEG while judging the brightness, since new compression artefacts can confuse the comparison.
Return to the source for another attempt instead of repeatedly brightening downloaded copies. Check the final picture at its intended display size, where a moderate correction often looks more natural than an aggressive preview edit.