You can’t directly embed pictures in this forum.
You can add links that the forum software will interpret and display but there are restrictions on what you can link to, based on your forum Trust Level.
Most posters are Trust Level 2, and at that level (or TL1) you can only link to a few specific web sites that Blizzard has decided to allow. Image posting services like Imgur are not among those trusted sites.
If you are active enough to attain Trust Level 3, the linking restrictions are lifted and you can link pretty much arbitrarily. Of course, linking to inappropriate sites is a fabulous way to completely lose all forum posting privileges.
Here’s an MVP post about this forum system. Search down a bit to find a description of the Trust Levels.