Screen shot your graph (edit it in paint or whatever if needed) and save it to a .jpg - or use the 'save image' function in e.g. Poker Grapher. Upload the jpg to imageshack, photobucket, or your hosting site of preference. Try and make the image not too enormous so it fits nicely into the forum thread without stretching things. Note the URL for that hosted image and use image tags to inline it in your post.
See this FAQ entry for more:
http://forumserver.twoplustwo.com/faq.php?Cat=0#image