If I use safe mode on your forum and I disable only the themes/theme components, as a regular user, here’s what the composer looks like:
The image icon has the same behavior as the 
icon.
So, the issue comes from one of your themes or theme components. Maybe one of them contains an error or it is misconfigured.
And I suppose one of your plugins alters the formatting bar as well.
Or maybe there is some incompatibility between a plugin and a theme component.