The images are converted to links to the original images, links which cannot be viewed in Word 2013, so you need a two step process.
First save as Word 97-2003 DOC format (in a folder other than that containing the HTML files), then run the following macro to unlink the images:
Code:
Sub UnlinkImages()
Dim ofld As Field
For Each ofld In ActiveDocument.Fields
ofld.Unlink
Next ofld
End Sub
Then finally save as DOCX with the images embedded.