easiest way for me is:

take the screen shot (press print scrn on your keyboard)
paste the image into paint
save the image as a jpg
upload the image to http://imageshack.us/
copy the direct link they give you of the image
click the image icon in your post and paste the link and it should work

Or you could just save the image to your hard drive and uplaod it to your post using the "manage attachments" button.

Yes, the easiest way is to save it to your HD, then click on Post a Reply (not Quick Reply), then there's a checkbox down the page where you can choose to add an attachment.

Also, alternatively, you can simply copy the image onto your clipboard and copy and paste it in.