You should first go to this page
http://dcf.outducks.org/contrib/upload.php
NB: there is a link to this same web page at the bottom of (nearly) all pages on the DCF website, named "upload tool".
Select the image file you want to upload and click on "upload". You will then see something like:
You simply need to copy & paste the bold text into your message, where you want your image to appear. The image will show as a clickable thumbnail (with the click leading to the full resolution image). Common supported image formats are PNG/JPEG/GIF.
Using the DCF upload tool ensures that images stay available over time (something that is not granted on other popular image upload services). That said you are free to use external images websites if you like.
Obviously make sure that you have the legal rights to distribute your image(s) through this forum.