Picasa Web is an online image organizing & photo sharing website. It allows you to edit, organize, sort and share photos into private and public areas as well as to your blogs. It is very easy to use and convenient way to upload photos to Picasa Web Album from Mac. You can upload photos and videos to Picasa Web directly by clicking Upload button. It also allows you to download photos, videos and an entire album from Picasa Web to Mac. To download an entire album, you first need to download and install the 'Picasa Desktop Application' on your Mac. There are 2 options to download an entire album i.e. from the Download menu and from the File menu.

Here are steps to Download an Entire Album from Picasa web to Mac:

1. Download & install "Picasa desktop application" on your Mac & launch it.
2. Go to https://picasaweb.google.com/ & sign in to your account.
3. Picasa offers 2 options to download an entire album.
  a) From the Download menu
  b) From the File menu

a) From the Download menu:
  - Select an album which you want to download.
  - Click "Download menu" & select "Download to Picasa".
  - A pop-up window opens-up, select "Save to Disk".
  - Your browser prompts you to save .zip file, then click "Save" and specify a location to download. The downloaded     zip file will contain the photo from the Picasa web.

b) From the File menu:
  - Click "File menu" button & select "Import from Web Albums...".
  - Uncheck "Import all albums not currently on this computer (Uncheck to choose manually)" option.
  - Select your album which you want to download.
  - Click "Ok" button.

4. Select any ('a' or 'b') one of the options listed above.
5. This is how you can download an entire album in Picasa desktop Application.

You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.