To upload your photos to your Photobox account using Picasa, open the Picasa program on your computer and click "Tools", then "Configure buttons...". Now, select the "Order prints" button from the list and click "Add".
A new button will appear at the bottom of your screen, saying "Shop". After selecting the photos you'd like to upload, click on this button. You'll see a pop-up, where you can select the website you'd like to upload your photos to: simply click "select" below Photobox. Now you'll be prompted to log in with your Photobox credentials, and then your photos will start uploading. When the upload is finished, a browser window will open where you can see your uploaded photos in your Photobox album.