JPG does not support transparency
When a transparent PNG becomes JPG, transparent areas must be flattened into a normal background. That can be fine for photos but risky for logos or cutouts.
When to keep PNG
Keep PNG when the transparent background matters, when the image has text or sharp edges, or when it is a logo, icon, or UI asset.
When JPG is okay
Use JPG when the image is a photo, transparency is not needed, and smaller file size or broad compatibility matters more.