You can't. We have that functionality disabled. Disk space, liability, and all sorts of headaches come with hosting images.
Therefore, the image will need to be hosted elsewhere.
There are plenty of image hosting platforms out there, but one that I've been using and is solid is: https://postimages.org/
It's free (with premium options) and I've never had a problem with them. They also have ready-to-post BBC code for forums like this. You can upload from your computer, or post a URL from another source and it will pull the image into your account.
If you have any image hosting solutions that you prefer, post it here.