Migrating your Dropbox photos to Google Drive account is easy using PicBackMan migrate tool.
Here’s how to migrate Dropbox photos to Google Drive
Note that PHOTO and VIDEO upload shouldn’t be running. Also, it might take time for migration to complete depending on the number of photos you have in your Dropbox account.
Here’s the video tutorial you can refer to
PicBackMan is a free photo/video uploader that helps you to keep your memories backed up to your favorite online services.
Users in 130+ countries have backed up.
... an excellent job of uploading more then 300GB of photos to 2 different services