There are most likely some memory issues here, if you have very large images it is probably eating up a lot of memory and bombing out before the import is complete. I'm not sure there is really a solution to this, as you are limited by the hardware on your host's server. In plog-globals.php there is a statement that says "set_time_limit(0)", which is supposed to allow indefinite script execution. There may be a setting in your installation of PHP which isn't allowing this to properly take effect. Sorry, I don't have a better answer. You can also group images into different folders within the uploads directory, but that might not be the time saving solution you are looking for.