Before reading this, make sure that you must be Jr.member or above the rank of Jr.member to upload images.
However, there is a way to upload images although you are neither Jr.member nor above.  If you pay the fee, you become a Copper Member, and you can post images.
You can buy it here: https://bitcointalk.org/index.php?action=credit;promote

If you are above the rank of Jr.member or Copper Member, You can follow this to upload images :

1. Go to https://imgur.com/    

2. Click on New Post



3. Upload your images



Then copy/paste your image link between two img tags

You can get sth like a bitcointalk code :

∨ -> get share links -> BBcode



I hope this is helpful to beginners