There are a number of easy ways to add more photos to the IM window when you're sharing photos:

• Click "Add Photos" in the side panel, beneath the pane where photos are displayed. This will open the Photo Sharing dialogue box, which lets you browse for photos on your hard drive. You can shift-click in a list of image files to add a range of files at once, or Ctrl-click repeatedly to select multiple files that are not in a continuous range.
• Or you can drag photos from a folder on your desktop to the side panel of the IM window. You can shift-click to add multiple photos at once.
• You can also drag-and-drop image files into the conversation area of the IM window. An inline notification will appear, showing that you're sharing the files with the other person. But if you drop only images (and not written documents or other kinds of files), you will also see a link that says "Add to Photo Sharing," allowing you to add the photos to the side panel instead of sending them via file transfer.