I just spent 20 minutes looking around the settings to see if I had misconfigured something. I'd like to hear the solution to this also.
I also noticed that the Image thumbnail limits don't seem to work on documents. I tried setting the limit to 100px but the images are displayed it's native 300px size.
It looks like Clearspace (not X) defaults to allowing registered users to upload / use images, but Clearspace X does not. You'll have to grant the permission explicitly: Admin Console --> Spaces --> Permissions --> Space Permissions --> check the 'create image' permission for the group of users you want to grant permission to.
Thank you! I did not notice this.