Pictures aren't 'in' or 'onto' Picasa. They are simply on your hard drive and Picasa is the program you use to access them. So you don't need to involve Picasa at all to send the pics by email.
You state that you received the pictures as attachments. If you've not already saved them to your hard drive, that's the first thing to do. (If you're not sure how to, see here:
http://www.webdevelopersnotes.com/tips/windows_live_mail/windows_live_mail_attachments_open_save.php ). You can save them in an existing folder (such as 'My Pictures') or create a new folder of whatever name (and in whatever location) that you choose.
Now create a new email to the friend that you'd like to see them. Windows Live Mail has an 'Add Photos' button but I suggest that you
don't use it. (It automatically shrinks the file sizes of large images which isn't really necessary with modern broadband download speeds and renders them useless for printing out). Instead, click on 'Attach'. Navigate to the location of the folder where you've saved the pictures to. Hold down the Ctrl button while clicking on each of those that you want to forward. When you've completed your selection, click 'Open' (or possibly 'Attach' or something similar). Then send your email as normal.