This facility is currently available in the Wiki TinyMCE editor where you can insert images and links to documents that have already been uploaded to the Wiki's associated site. These buttons are added in the "components\wiki\page.js" by the "alfresco-imagelibrary" and "alfresco-linklibrary" values in the configuration options used to instantiate the editor. I don't know whether there are plans to introduce the same capabilities into the blog and discussions editors but it should be possible for you to customize share to add this capability yourself. I haven't investigated this any further, but this should give you enough information to get started if you want to add the capability yourself.
Regards,
Dave