Welcome to the Apple Discussions. Select the photo you want to change, go to the Inspector/Graphic pane, click on the drop down menu under stroke and select Picture Frame. There you'll find many frames to choose from.
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If you want over 80 more frames that are hidden in iWeb but available to choose from with some simple effort try the following:
The standard IWeb, Pages and Keynote applications have only 24 frames for applying to photos. This application changes the preferences files for those applications to make available nearly 80 additional frames:
Frames and strokes installer for :iWork ’08, iWork ’09, iWeb ’08 et iWeb ’09.
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Be sure to keep the install package as upgrades to iWeb, Pages or Keynote, and possibly updaters, will remove them and you'll have to reapply it go get them back..
OT