It is possible to add photos from Imgur without signing in. Other sites are available.
On the Imgur site [imgur.com], under New post, click on 'upload image' then select and upload your image(s).
Under the 'Share this Image' header copy the BB Code(forums) text and paste it into your STF message. When you are in "Post a new topic" there are a series of icons above the box - the link one (10 from left) allows you to post the BB Code in that, between the two URL.