Okay well I offer no support or warranty this is posted for your personal use AS IS.
With that said make sure before you do anything back up your data directory. You should have backups as part of general website maintenance but this script should work.
Basically you need to lower the max height and width in upload photos to a lower size if you want to lower image sizes. You can then rename this script to resizephotos.php and then run it from your photopost directory
In theory this should work.