Why does my logo have a white edge on a dark photo?

Because the edge pixels still hold some of the white it was cut from. Try Decontaminate at 1 px on the Mask tab; if a ring remains, add a thin dark outline on the Finish tab.

Will Decontaminate make my logo smaller?

By at most two pixels at the edge, which on a logo a few hundred pixels across is not visible.

Why does my signature have a grey edge?

Ink and paper mix at the edge of every stroke. Raise Edge softness a little, then Decontaminate; recolouring to one ink also helps because the edge then fades from your ink, not from grey.

Reading

Why transparent PNGs get a pale fringe

A logo lifted off a white plate, a person cut out of a bright room, a signature photographed on paper: each arrives with a thin ring of the old background. Where it comes from, and what each tool on the stage can and cannot do about it.

Open the placing toolBoth files stay on your device — we only draw one on top of the other.

Where the ring comes from

The pixels along an edge are never purely the object or purely the background — they are a mix of both, because a camera pixel covering a boundary sees some of each. When the background is taken away, those border pixels keep some of its colour. On a white plate that means a pale rim; on a green screen, a green one.

On the background it was made for, the rim is invisible, because it matches. Drop the same PNG on a dark photograph and the rim is suddenly the brightest thing on the edge.

What Decontaminate does

Decontaminate, on the Mask tab, does two things. It eats into the edge by up to two pixels, so the outermost and most contaminated ring goes. Then every pixel that is still part-transparent takes its colour from the solid pixels just inside it, which replaces the old background's colour with the object's own.

It helps; it does not always fix it. A fringe wider than two pixels is in the source file, not in the edge, and eroding further would start eating the object.

What the other tools can do

An outline on the Finish tab draws a ring of your chosen colour round the whole shape. A dark outline hides a pale fringe completely, at the cost of looking outlined. The colour key's Edge softness widens the fade at a keyed edge, which turns a hard pale line into a soft one. And for a cut-out made on this page, Grow or shrink the edge pulls the mask in by a few pixels before anything else happens.

In short

Decontaminate: 0–2 px of erosion, then border colour taken from the solid pixels beside it.
Outline: Any colour, any width, drawn outside the shape.
Colour key softness: Widens the fade where a keyed colour meets the object.
Cut-out refinement: Grow or shrink 4 px either way, feather up to 8 px.

None of these can recover hair or fur that the source flattened into the background.
A PNG exported with a matte colour baked in is better re-exported without one than repaired here.

Questions

Why does my logo have a white edge on a dark photo?: Because the edge pixels still hold some of the white it was cut from. Try Decontaminate at 1 px on the Mask tab; if a ring remains, add a thin dark outline on the Finish tab.
Will Decontaminate make my logo smaller?: By at most two pixels at the edge, which on a logo a few hundred pixels across is not visible.
Why does my signature have a grey edge?: Ink and paper mix at the edge of every stroke. Raise Edge softness a little, then Decontaminate; recolouring to one ink also helps because the edge then fades from your ink, not from grey.