PHP is set to a max upload size of 2mb so if your trying to upload a file larger than this your going to have problems with this as your server will drop the file so there is nothing to process. The issue is from your PHP and not Photopost.
Only way around that is to edit the php.ini file and change your max file uploadsize higher but you may not be able to do this.
You may be able to place this below in an htaccess file in your web root an that will raise the limit but not all servers support that
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