Transparent backgrounds: PNG keeps them, JPG flattens them
PNG stores transparency; JPG cannot. When a transparent PNG is converted to JPG, the transparent areas are filled with a solid background color.
What transparency is
Transparency lets the background show through part of an image. Logos, icons, and cut-out product photos use it constantly, which is why they are usually delivered as PNG.
JPG stores only red, green, and blue color values with no alpha channel, so it has no way to represent a see-through pixel.
How JPG handles transparent pixels
When a transparent PNG becomes a JPG, the converter fills the transparent pixels with a solid color, usually white. Rounded logo corners or soft shadows can therefore end up inside a white box.
That is fine on a light background and invisible in most photographs, but it stands out on dark pages or colored layouts.
Choosing between PNG and JPG
Keep PNG whenever the image may sit on an unknown or changing background. Convert to JPG only when you control the background and a smaller file matters more.
When you do flatten, flatten the image onto the final background color first so the result looks intentional rather than like a white box.