JPEG is a common choice for photographs and application uploads. Its compact files come from lossy encoding, and it cannot carry a transparent background.
Use Convert Image to JPEG when the destination explicitly needs a JPEG. Set the quality and the colour that should appear beneath transparent source pixels.
The canvas size stays the same
A 400 by 300 PNG becomes a 400 by 300 JPEG. Converting the format does not resize the source or force it into a different ratio.
If that PNG has a transparent corner, the JPEG fills it with the selected background. White is useful for many document style images. Another colour may be needed for a design asset that will sit on a coloured surface.
The quality value controls an encoding tradeoff. A higher value usually retains more visible detail and can increase bytes, but it does not restore detail already lost in the source.
Look closely at text and sharp edges
Compression marks are often easiest to see around lettering, logos and narrow high contrast lines. A photograph can look acceptable while a scanned signature at the same quality looks rough.
Avoid repeatedly saving an already compressed JPEG through new lossy copies. Keep the best source and generate another output from it when adjusting the quality.
The converter removes original metadata and profiles during reencoding. That can simplify a basic upload, but a colour sensitive publishing workflow may need the original profile retained elsewhere.
After downloading, check the reported dimensions and byte size against the receiving form's rules. A file with a .jpg extension and valid JPEG bytes can still be too large, too small or framed incorrectly for that particular application.