Free up phone memory taken up by thumbnails I have a 16 GB Samsung Galaxy S3 phone - 5.05GB of which is being taken up by thumbnails! Can I move these to my 32GB SD card? 

I have tried DMR (duplicate media remover) and that has detected duplicate pics and videos but stops when I try to delete them.
You can try using an app like Es File Explorer

It has a SD Card Analyze tool, which will help you analyze the total space used and give you options to clear some space by deleting unused files, temp file etc.

For the issue with thumbnails, take a backup of all your photos on Google Photos.. then delete all the thumbnails.. deleting thumbnails will not delete the images.. Even if it does delete, you will still have the backup in Google Photos.