Why Transparency Disappears in JPG
PNG can store an alpha value for every pixel. JPG cannot. During conversion, a transparent edge must be blended with a solid color, otherwise the result can appear black, dark, or fringed when displayed against a different background.
This is most obvious in logos, stickers, and cut-out products. Semi-transparent antialiased pixels around an object were designed for the original background, so choosing a new background is part of the conversion—not an afterthought.
Choose a Background Before You Convert
Use white for government forms, standard documents, and marketplace listings with white product backgrounds. Use a brand color when the final image will always sit on that brand color. For photography, choose the color that matches the destination page or crop away transparent padding before export.
Check the result at 100% zoom. A light edge on a dark background, or a dark edge on white, usually means the original transparency was already matted against another color.
Converting a logo to JPG rarely improves it. JPG can create ringing around sharp letters and removes the transparency that made the logo reusable.
When JPG Actually Helps
JPG is usually much smaller for photographs, gradients, and full-bleed images. A phone photo mistakenly saved as PNG can shrink dramatically as JPG with little visible difference.
Keep PNG for interface screenshots, diagrams, line art, and icons. If delivery size matters, resize the longest edge before reducing JPG quality; that approach often preserves detail better.