Shift, control, command, 4.
Using your left hand, can you press all four of those keys simultaneously?
If you can (it might feel a bit uncomfortable at first), you'll save yourself a ton of time when wanting to use graphics from one application and put them into another one.
Do you have a picture from a web page you want to show someone? Press those 4 keys, drag a marquee around the image you want, and then listen for the camera "shutter" sound. The image then goes into your system's CLIPBOARD: it's not saved onto your desktop. That's usually fine for most people because they don't need the file on the desktop, they need the file to GO somewhere. Once it's on your clipboard, you can simply PASTE it into whatever application you're needing to use. This works in Slack, Word, Facebook, graphics applications, and many others.
Apple barely highlights this clipboard option on their web page: https://support.apple.com/en-us/HT201361 and once you learn it, you'll use it all the time.
You'll find you accumulate less clutter of "needed it just momentarily but no longer" types of files. Try it out!
p.s. Windows users, you have options too: https://www.windowscentral.com/how-use-new-clipboard-windows-10-october-2018-update