A transparent PNG can show a subject, logo, or object with no visible background. Convert that file to JPG and something has to change, because the JPG format has no alpha channel for transparent pixels.
The converter must replace transparency with real color before the JPG is encoded.
JPG does not support transparency
PNG and WebP can store an alpha value alongside color information. That alpha value tells the browser whether a pixel is fully opaque, fully transparent, or somewhere in between.
JPG stores color information without an alpha channel. There is no valid JPG instruction that means “show whatever is behind this pixel.” Once the file becomes JPG, every output pixel needs a visible color.
Why transparent pixels need a background
Before encoding a transparent image as JPG, the image is composited over a background. A fully transparent pixel becomes the background color. A partially transparent edge is blended with that color.
This matters most around anti-aliased edges, shadows, hair, glass, and soft cut-outs. Choosing the wrong background can create a visible halo even if the original transparent image looked clean.
imgevoke's Image Converter uses white as the launch default for JPG conversion and provides a background color control so the choice is explicit.
Which background color should you choose?
Choose the color that matches the place where the JPG will actually be used. A product image intended for a white ecommerce page usually works well on white. A graphic for a dark presentation may look better composited against that dark background before conversion.
If you do not know the final background, converting to JPG may be the wrong operation. Once transparency has been flattened, the original alpha information is not present in the JPG output.
Keep the transparent source file separately if you may need it again later.
When PNG or WebP is better
Stay with PNG when exact lossless pixels and transparency are important. Consider WebP when you need transparency but also want a modern format that can offer smaller lossy output for many images.
Use JPG when transparency is unnecessary and photographic compression is more valuable than retaining an alpha channel. The best choice comes from the destination, not from the idea that one extension is universally superior.