I would upload whatever picture onto IMGUR.com
On the upper right corner you click "+ NEW POST"
Click "CHOOSE PHOTO/VIDEO"
Once you have uploaded your picture. Move you mouse cursor over your picture. Right click your mouse and and click "COPY IMAGE ADDRESS". My browser is Chrome but Internet Explorer, or other browsers are similar.
Now, come back onto AR15.com and go into a threat, click reply.
When you are typing your reply, look at the message options to the top. You see that chainlink icon? Right next to it is an icon of a mountain and sun, click this icon.
You will prompt a new box that says "INSERT IMAGE". Now right click, and paste the link you copied on IMGUR.com
You image should post in the thread.