Automatically Save Screenshots (Print Screen) as JPEG

What exactly do you mean with automatic? I can see at least three meanings.
1. It makes the screen shot automatically, for example each minute.
2. It makes the screen shot if you press some hotkey, but safes it automatically as JPG.
3. You need to give some command to save it, but it defaults to jpg, so there is no need to change the default.

If you mean 3. paste the screenshot into irfanview (free). It has jpg as default, while Windows' Snip and Sketch defaults to png.

While the default is PNG, it can be set to something else.
More at https://itectec.com/superuser/automatically-save-a-snipped-screenshot-to-disk/

Click File
Select Save As
Choose a Filename.
Select the drop-down beside the "Save as" type.
Select JPEG file.
Press Save.

I hope it helps