Normally you can use the Desktop uploader from Google Photos to upload any folder with photos on your PC to Google Photos.
- Install Dropbox
- Install Desktop Uploader
- Setup Desktop uploader to sync your dropbox/Pictures folder
Unfortunately Desktop Uploader is only available for Mac and Windows.
But the solution cannot be easier.
- Go to https://photos.google.com
- Drag your dropbox picture folder in the browser
- Profit
