Picasa web is a photo organizing and sharing website operated by Google. Picasa web is now integrated into Google+ account. If you have a Google account, then you can directly access Picasa Web. Google+ has a 'Auto Backup' feature which allows you to automatically backup photos and videos. You have another option to upload photos i.e. 'Drag-and-drop'. Here you can simply drag your photos to a location in your Picasa web account from Mac. You can also upload photos and videos manually from your Mac with simple steps.
Here are the steps to Upload Photos from Mac to Picasa web (If you have signed up for Google+):
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.