If your images are showing up with 0 height/width you have likely run into a PHP bug. There was a bug introduced in PHP 5.6.24 that causes the image size process to fail for some images (i.e. generating the alternate sizes specified in Settings -> Media).
If you can roll back PHP to 5.6.23 or forward to 7.0.x that will fix the issue.
If you don’t control what version of PHP is on your server you can change the image template Postie uses. Go to the Image tab in Postie settings and look for the Image Template setting.
The default is:
Change it by removing the height and width attributes:
For those interested the bugs are listed here: https://bugs.php.net/search.php?cmd=display&package_name[]=EXIF+related