If you do upload the picture to a file-uploading site, you'll have to read their Terms and Conditions. A lot of sites like that claim ownership of anything that gets uploaded to them, so be careful.
It's probably a lot simpler and easier to just save it to your pictures folder on your computer and then attach the picture in your post. It's not difficult at all. You click "reply" then click "attachments and other options" which is right below the text box. Then click "browse", select your picture, press ok and then submit your post.
It's really REALLY easy, and it's just generally simpler and safer than submitting it to a file-uploading site.