Mac screenshots by default save in .png format. Would you prefer the default format to be JPG, or something else? Here's the simple way to change it.
From Finder, launch Terminal:
top menu Go > Utilities > Terminal
Copy and past in the following line, it's a single line with no break:
defaults write com.apple.screencapture type jpg;killall SystemUIServer
Want a different format? In this line of code, replace jpg with one of the following: