Gum:
I assume you know how to save a graphic. If not, it goes like this: Right-click the link for the item you want, and then click Save Target As.
When you save the graphic, make sure it is in the form of: gif, jpg, png, txt, zip, bmp, or jpeg.
When you click on "Reply" and the screen appears to start writing, scroll down to the bottom of the screen. You will find a section named "Attach File" with a little box next to it with a "Browse...." button beside it. Click on the "Browse...." button, and locate the file that has your graphic. Then click "Submit".
I assume you know how to find a file in Windows Explorer.
That's it.
PS: I think they just changed the board to allow you to do this. Another board I go to has the same message board program and they just modified it. Before, apparently you had to get a web page, upload the graphic to your web page, then put the URL on the reply box. What a hassle!