If you're using the new editor, this isn't available at this point - vote / watch for this update on CONFCLOUD-65640
You have a few options in the meantime:
Hi Stephen, hi everyone,
I tried to use the legacy editor and was able to put links on the images but other users then me and the other space admins cant see all images. They see an info "unknown attachment", see screenshot. Any idea why? Thanks.
I'm not a confluence admin and don't have the permission to install plugins and also would prefer not to install a plugin for adding a link to an image.
A simple check is to go 'Insert files and images' on the top-bar and check the attachments attached to this page.
An option here is that the image is not uploaded to this specific page, but another somewhere else in the space - and that page is restricted. For example, if Image 1 is attached to restricted Page A and you refer to it again on Page B, users without view permission on Page A will not see it properly.
If this doesn't work - I'd suggest you contact https://support.atlassian.com/