It's easiest to do on a mac. When on a mac, click Command+Shift+4. This let's you choose a region to take the picture.
For PC users, they can use a system like lightshot or gyazo. This program has the same function as what I mentioned above. They can also press Prt Scrn, or Print Screen. This copies an image of your desktop to their clipboard, which they can then crop to fit.
Once the image is satisfactory, you can do one of two things to add it to a post or signature. First thing you can do is upload the image to imgur.com. Once it's uploaded click this button which is in the toolbar:
Copy and paste the link of the uploaded image into the box and the image should appear.
Second, click this button:
Then choose the image you want to upload.