Be sure what you want to take a picture of is front most on your desktop
Open the Snipping tool pined to your desktop
Click the “New” Icon
Drag a box of what you want to capture. A new window will open with a jpg of your area you selected.
To Copy that picture, press Ctl + C
Open slack and go to the person where you want to send that picture. Click in the Text sending box
Press Ctrl + V
Click “Upload”