Locate the "Image" icon in the horizontal bar of icons just above the space you type your post, and select it:
A new pop-up should appear. Select the "Upload" tab in that pop-up.
Click on the "Browse.." button and then locate the image you want to upload, on your device, and select it.
Back to the pop-up, select the "Upload" button.
Back to details, you can *optionally* include a description for people that cannot see images (blind, browse does not support image) but "alt" details can be read, and then click "OK":
Your DEF CON forum post should now have that image included as an attachment.
For example.
If I were to write:
"Here is an image showing Dark Tangent standing next to a PDP-11" then included an image of that after that text, then there is no need to include that text in the image "alt" description, as it would be redundant. It does not mean you should not, just that the purpose of the alt/description of the image are described in text before the image, then your atl/description is primarily met. (It would not be harmful to add text to the alt/description section, and would likely help search engines that use alt/description text for hints in categorizing the image.)
If I were to write: "LoL! So funny!" and then include an image, then it would be a HUGE benefit to others if you included text in the "alt/description" of the image to describe it, so people unable to see images can understand your comment.
Social systems are about communicating; if you can increase the reach of your message to a wider audience, you reach a wider audience. :-)