Siimply upload your image, copy the code that comes up on screen and paste it into the message box at the Avant support forums.
Thats it, you have now inserted a screenshot in your message.
If you wish to post it as an actual image rather than just a link, simply enter the following into the box (replacing THE_URL with your image URL as mentioned above).
[img]THE_URL[/img]
*Hint: Try pressing ALT along with Print Screen to just take a screentshot of the active window
Note: Images posted on the Avant Browser forum MUST be a maximum 800x600 pixels (height/width) and the file size should be no more than 300K (to prevent possible inconveniences for dial-up users).