The insert image buttom in the WYSIWYG should be configured so as to allow users to select images from uploaded files. The selection populates the Image URL and serves as a shortcut for the user.
See:
http://screencast.com/t/mQsU8Pjvr
And:
http://screencast.com/t/Z1jKkp7zMQD
Official response from Jim Gilliam
It does include the images that are uploaded to the page, but not the images uploaded to the theme. The problem with including theme images is that you could change the theme and then those images wouldn't work anymore.
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Do you like this suggestion?
Oh it looks like it does do this for files associated with the content item. My suggestion is that it should also look in the library of files uploaded to the theme.