A practical guide
Flatten transparency deliberately, not by surprise.
JPG is widely accepted for photographs, email attachments, office documents, marketplaces, and older publishing systems. Converting a PNG to JPG can be useful when transparency is unnecessary and a photographic image is too heavy as lossless PNG. PixelShrink performs the conversion locally and lets you choose both JPEG quality and the solid color placed behind transparent pixels.
That background choice is permanent in the downloaded JPG because JPEG cannot store alpha. White is the default, but a page-colored background may prevent a visible box around a logo or cut-out image. For screenshots, diagrams, and small text, lossy JPEG artifacts may outweigh any size saving; compare the processed preview closely before replacing the PNG.