What a transparent image actually stores
Transparency is pixel data
An alpha channel records how opaque a pixel is. Fully transparent and partially transparent pixels can exist alongside ordinary colored pixels.
A plain color is not empty
A white background in a JPEG is ordinary image content. Converting it to PNG cannot know that you wanted it removed. A transparent source is needed to preserve transparency.
Inspect edges on two backgrounds
Light and dark backgrounds can reveal pale halos or leftover matte colors near the artwork. Those issues may come from the source. A format converter keeps that decoded appearance rather than redesigning the image.
Try the workflow
A transparent WebP can become a PNG without adding a solid background. A JPEG will remain opaque because it has no transparent pixels to preserve.
Open png converter