[DerekM]

Currently only images that don't have any spaces in their name will work. Some special characters are not allowed either. You'll need to drag the pasted images out to Finder, rename them, and then add them back in. OmniOutliner currently doesn't have the ability to rename attachments to satisfy the docx format.

Or, if you have Pages, you can try importing the docx file to that and then re-saving as docx and it will rename the images for you. Pages is much more relaxed about importing the file format and shouldn't have any issue with spaces in the file name.