A JPEG is usually smaller than the same picture as a PNG, which helps when a site limits upload size. Rename the file in Finder and Renamable converts it right where it is. Nothing is uploaded.
PNG.png
JPEG.jpg
Screenshot.pngScreenshot.jpg
Yes
How to do it.
Select Screenshot.png in Finder and press Return.
Change .png to .jpg and press Return. If Finder asks, choose “Use .jpg”.
Renamable converts it right there. The original goes to the Trash, or stays next to the new file if you prefer, and you can undo any conversion.