I've been asked to provide an update. I have had the suggestion that I can add padding to the bound object frame to centre the pictures that need centring, and been given some code that could get me started on testing that possible solution.
However, I have reason to think that using a later version of Access would enable me to go back to using jpegs in image controls, and yesterday I found that the group that I'm making the database for is actually using a later version of Access.
As soon as I have the time, probably before the end of the week, I am going to see the lay of the land in Access 2010, with regards to this.