Follow this steps to reproduce:
- Edit a page in WYSIWYG mode
- Switch to source and set the content to:
- Switch back to WYSIWYG and double click the image place-holder
- Choose Edit Image from the Image menu.
The value of the space selector is 'Blog' and the page selector is empty. You can get the same result if you try to insert an image from the All pages tab while editing a new page (that hasn't been saved) in a new space.