The editor is a visual editor, you cannot insert HTML. You should click the {;} button to see and insert HTML. However, it's better to insert the image URL in the visual editor, it'll be turned to image tag automatically once the post is submitted.

If you want to have an advanced file attachment options, you should check out the wpForo Advanced Attachments addon: https://gvectors.com/product/wpforo-advanced-attachments/

At present I can only add one attachment at a time but would much rather be able to embed an image into the post.  On my old forum I would use this text (generic link from html website) <img src="img_girl.jpg" alt="Girl in a jacket" width="500" height="600">  However when I try that on wpfor I get a small icon instead (like the image missing logo in screenshot).

The forum I run is for hair photos so I need for people to be able to easily add photos to their posts.

Sprry for the 'probably obvious' questions, I am searching the forum first but am new to this (recent smf convert). :D