Well that the issue has nothing to do with Photopost or your comupter then now that you gave more info.
I figured you where uploading the same types of files on both computers
Your server's php maximum filesize is limited to 2MB and I bet the files your trying to upload are over this size.
You can edit your server's php.ini file and change the max fileupload size
If you can not change this then in edit usergroups make sure to set everyone's max filesize to a safe number like 1536 so they get an error message from the program instead of the nasty server die message of PHP