open the pdf in adobe reader. Then from the Edit drop down menu select "copy file to clipboard"
Now open Paint and from it drop down menu select paste (or just hit Ctrl V)(or right click and select paste)
you will most likely have to adjust the background size to fit the image.
then save as a jpeg file (.jpg).
Now you need to upload your image to a host. My favorite is tinypic
after you upload the file to tinypic it will give you several links you can copy. find the direct image link and then when you post to the forum just hit the Insert picture button and paste the url into the pop up box. Or to put it in manually use the bbcode
If you have the full blown Adobe acrobat I think you can just save as a .jpg by going to file--> save as--> and then selecting the file type you want to save it as.