Just a quick tip, which I'm putting primarily to never forget to change the default behaviour in MacOS.

tl;dr

defaults write com.apple.screencapture type jpg #change output type to jpg  
defaults write com.apple.screencapture location ~/Pictures # change output folder  
killall SystemUIServer # kill to see changes  

Changing filetype of captured screenshots

Open terminal emulator of preference and execute
defaults write com.apple.screencapture type jpg to change the output filetype to jpg. To reflect the changes without restarting kill the systemUI server via killall SystemUIServer

If anyone might be wondering why they should bother, here's the size comparison of a full-screen screenshot (Retina MacBook Pro 2015, native 2560 × 1600.)

comparison

Changing location of captured screenshots

By default, macOS throws the screenshots on your desktop. To alter the output folder use:
defaults write com.apple.screencapture location <path_to_folder>

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