Taking a screenshot is super helpful and there are many ways to do it. My go to on the Mac is the keyboard shortcut
cmd+shift+4. This brings up a crosshair to select which part of the screen you want.
When you're done selecting you the image will be added to wherever you've configured screenshots to go. Normally the default is directly to your Desktop.
To add this screenshot directly to your pastebin you can do an alternate shortcut of
cmd+ctrl+shift+4. Then you can just paste the image wherever as soon as you're done. This is awesome but is a hard keystroke for me. However I discovered that you can use the original
cmd+shift+4 to activate the crosshair but it can be redirected to the pastebin if you hold
ctrl before you click to finalize the screenshot'd section.