You should use LinkManager and ImageManager dialogues that come with the RadEditor to import links and images with relative path.
In order to enable them you need to modify the ToolsFile.xml which is located in the following folder:
/Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/188.8.131.52__1f131a624888eeed/Resources/
Just add these lines
| <tool name="LinkManager" />
| <tool name="ImageManager" shortcut="CTRL+G" />
in the root node.
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