Transparency lets an image reveal content behind it instead of drawing an opaque background. In raster graphics this is commonly represented with an alpha channel, which can describe full or partial opacity.
Formats that support transparency
PNG is widely used for transparent graphics. WebP and AVIF can also support transparency. JPEG does not provide an alpha channel in the conventional format, so transparent areas must be flattened against a background color when converted to JPEG.
Why edges can look different
Partially transparent pixels around logos, hair or shadows are blended with whatever appears behind them. If those pixels are flattened against white and later placed on a dark background, a light halo can become visible.
Choosing a target format
If transparency is essential, choose a format that supports it and verify the result. If the final asset will always sit on a known solid background, flattening can be acceptable and may simplify the workflow.
Conversion cannot invent transparency
Converting an ordinary JPEG to PNG does not automatically remove the background. The new PNG can support transparency, but the original pixels still contain the old background unless a separate background-removal process is used.