As far as I know from Picasa it's only possible to do a bulk back up to DVD. This is in the 'tools' menu under 'backup pictures'. For USB or HD backup you would need to go through your file manager and drag and drop. If all the relevant folders are in one folder (e.g. 'pictures' or 'my pictures' etc) you can simply copy this folder to the backup. The data files that Picasa creates with a record of your edits is saved in the same folders as the images you have modified so even if you haven't exported these images the changes you have made will also be backed up.
Not certain if this is the answer you are looking for but if you right click on the folder you have open and select “All” then go to “Export” at the bottom you can then determine where the images are to be sent. I use an external hard drive for the destination.