Picasa is an image organizing & photo sharing website. When you add photos to Picasa Web Albums, they will also be accessed in Google+ photo tab. If you already use Picasa Web Albums, you will have access to all of your existing albums in Google+ photos because Google+ and Picasa Web Albums are now connected.
Following are the steps to upload photos from computer to Picasa web (If you have signed up for Google+):
1. Go to https://plus.google.com/ & sign in to your account.
2. Once you have signed into your account, click "Upload" button located at top of the window.
3. A pop-up window "Upload from computer" opens up.
4. Click "Click to open the file browser".
6. "File upload" window opens up. Browse & select photos that you want to upload.
7. Click "Add to album" button located at left corner of the window.
8. Create new album or choose an existing album to save your photos.
7. Click "Ok" or if you want upload more photos then click "Upload more" located at bottom right of the window.
8. You have another option to upload the photos i.e "Drag-and-drop". Here you can simply drag your photos directly from your computer to a location in your Picasa web account.
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.