If a photo has already been uploaded by you into your own account, or if you have already shared a photo, or if someone has shared a photo to you so it appears in your Received area, it can be shared again.

Touch the small thumbnail in the Received, Uploaded, or Featured area for the photo you want to share.

This will take you to the photo detail page where you will see a photo thumbnail, comments, etc.

Touch "Share" in the bottom of the screen in order to share your photo with friends and family. You will see icons to share to phone, Twitter, or Facebook.

Touch "To Phone"; you will see your iPhone contact list appear; select one or more users from the list or add a new user via the + icon. 

Touch "Share" to share your photo to family and friends. Your friends and family will receive a notification (SMS messages for mobile web users and push notifications for iPhone app users) for your shared photo.

Note: if you have created a list of friends with whom you share frequently, you can share to these people too by touching the "Thwapr Friends" icon and selecting the people to whom you want to share your photo.