For saving the document as template, choose "Save As" and under "Save as type" choose "PowerPoint template".

For locking the items use the function "Group" by selecting the items by holding down CTRL and selecting them with your left mouse button and after done, press with your right mouse button on the items and choose "Group".

Cutting off the images is unfortunately not possible in a easy way. If the way they are during the show is suitable for you I'd stick to that.

Hope to have helped.

Rgrds,
Kristiina