You can do the following: Rename the file from .docx to .zip. Open it with the Windows Explorer and navigate to the subfolder word/media. There you will find your pictures. Delete at least some of the big ones to get under 512MB.
Rename back to docx and open with Word. Word will display some errors, because it can't find some files, just click Yes to recover the contents.
After that you will find placeholders for the images you deleted. Right-Click on them, choose "Change picture", select (just SELECT!) the picture on your local harddrive (better copy it to a directory under the Word document). Beside the Insert button you will find an arrow, click it and select "Link with file". After that, your file won't get bigger, but you will need the pictures to be beside the Word file.