Pasting images into Pages document very cumbersome - Inline vs automatic
First make sure that the image you want to replace is defined as a media place holder.  To do this click on the image that you want to replace so that it is active.  Now go to format at the top of your screen and choose "Advanced" from the "Format" drop down.  You should see the following bold text with a check mark in front of it "Define As Media Placeholder".  If it is not bold with a check mark in front of it click on it so that it is.
Once the image has been defined as a media placeholder right click on it and choose "Replace Image".  You will be given a choice of file paths to choose your new image which should then replace the original image and keep its formatting.  This only hitch is that you might be limited to choosing only those images which have been saved to your photo stream.