I took a ton of pictures on my Lumia 930 over the last months, and I'd like to remove them to free up space. All pictures are backed up to OneDrive. I'd like to keep the 100 or so pictures I've marked as favorites on the phone. Neither OneDrive nor the 'Photos' app on Windows 10, nor the 'Files' for Windows Phone 8.1 shows this favorites status. How do I sync these specific pictures?
Yes, I just found a way to do it, quite simple albeit not very obvious:
I'm using the Windows Phone app on Windows 8.1 to sync my photos from the phone to PC. On the PC under Windows, every photo has an optional 5-star-rating in its metadata, which can be accessed/set e.g. via Explorer -> Properties -> Details tab. You can also add this property as an additional column in Explorer's detailed list view of files. Then you can easily sort pics in a folder by clicking on this column header.
Apparently the Windows Phone "favorite" rating translates into 4 stars in the metadata (while the rest has no stars). So you can simply select all 4-star-files in an Explorer folder sorted for the rating and copy/move those files away.