Can Mac Preview be set to default to save image as JPEG
No easy preference or key is known to change the defaults. Barring a new discovery, plan to use a different app or script image conversion with automation. 

You probably already know you could make a keyboard shortcut if an Apple menu to save as JPEG existed, but that’s the crux here. Other questions here have useful details on automation and using python to process the clipboard instead of preview. 


Is there a way to change the default save/export format in Preview?
How can the default save format be changed for OS X Preview app?