Convert PNG to JPG
The usual reason to do this is an upload box that only takes JPG, or a screenshot that is far larger as a PNG than it needs to be. JPG has no transparency, so anything see-through is filled in first — pick the colour below.
Questions
My last converter turned the transparent parts black. Why?
JPG has no alpha channel, so a transparent pixel has to become some real colour. A converter that skips that step hands the encoder zeroes, and zero in RGB is black. Image KB Reducer composites onto white first — or whatever colour you pick.
Is a JPG always smaller than the PNG?
For pictures and screenshots of pictures, almost always. For flat graphics, logos, and text screenshots, PNG is often smaller AND sharper — JPG's compression puts visible fuzz around hard edges.