You cannot lock an image that you've inserted on a slide.
You can password protect your presentation with PowerPoint 2002 or
2003. This prevents viewers from copying or editing your file.
Or, if you have the background file - you can merge the inserted image
with the background using a photo editing program (Photoshop, Photo
Paint, etc.) Use this merged image as your background for the one
slide. Be aware that this method is time consuming for multiple images
and/or revisions.
Julie