How to Change the Default Screenshot File Format in macOS
The default file format for macOS screenshots is .PNG, but that may wind up being more of a nuisance than you'd prefer. That's why I set my MacBook to take screenshots as JPGs, as those are better for our websites.
Here's how to change the default format for your Mac screenshots:
1. Hit Command+Space to open Spotlight.
2. Type "terminal" and select Terminal.
3. Ignoring the quotation marks, type "defaults write com.apple.screencapture type " followed by the desired file format. Your options are JPG, TIFF, GIF, PDF and PNG. Make sure to enter a space between 'location' and the format.
4. Click Enter.
Now, your screenshots will appear in this format. If they do not, restart your mac and they will.