how do i bring the Xthread field into the portal?

[RateU]

Assuming the post_image is an image custom thread field, there is an explanation under the Input Field Type setting when you create or edit the custom field.

The basic usage is by using {$GLOBALS['threadfields']['key']['item']} variable. Replace the key to your Key (in your case is post_image and the item with one of the list in the explanation.
So, for example, if you want to get the image url, you can use {$GLOBALS['threadfields']['post_image']['url']} or if you want to get the Display Format of the field, you can use {$GLOBALS['threadfields']['post_image']['value']}

But if the field accept multiple images, the format is slightly different (you can also find the explanation about it under the Input Field Type setting).