Picasa Web Albums is also known as Google Photos. If you want to upload a folder to Picasa Web Albums, then download and install "Picasa for Mac" on your Mac computer. Open "Picasa" and select the folder which you want to upload. Click "Sync to Web" button located at the top right of the page. The folder will start to upload and synced to Picasa Web Albums. When you click on "Sync to Web" button you will see the blue spinning wheel that means the upload is in progress. You will see a two curved arrows located next to the folder icon when a folder has been uploaded and synced. If you do any changes to a synced folder in Picasa, then it will also be made to the same folder in Google Photos.