Yes, but it doesn't store or upload it automatically. You can use Picasa photo uploader or any other third party Picasa uploader (like PicBackMan) in order to upload photos to Picasaweb. Some of these photo uploaders automatically upload photos from your computer to your Picasaweb albums without much efforts from your side.
Picasaweb works fine as a place store a backup of your photos, but its not a place to store original photos. Picasaweb make changes to your original photos. The photos that you upload to Picasaweb are compressed and re-size d to save space.
You ca upload photos to picasa through PicBackMan's Picasa uploader.
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.