Post Preview Image not using img_path attribute value

[dvdmtw98]

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• Read Wiki to understand the usage and the correct effect of functional design.
• Make sure no similar issue(including closed ones) exists.
• Make sure the bug is found in the latest code of the master branch.

Describe the bug

The Preview Image feature introduced in v6 does not use the img_path attribute when looking for the image

img_path: /assets/
image: 
    path: images/post-name/image-name.png

When the following structure is used the Banner image loads properly in the article. The img_path value is joined with the image path value to get the full path to the image.
But the same image does not load as the preview image as it expects the path mentioned in image path to be the complete path i.e. value of img_path is ignored/not used.

To Reproduce

Steps to reproduce the behavior:

1. Use the img_path and image path values together
2. Preview Image does not work

Expected behavior

The img_path value should be used to determine the path to use to look for the preview image

Logs/Screenshots

YAML attributes used

img_path: /assets/
image: 
    path: images/nat-explained/nat-explained-banner.png

Preview Image not Working

Working Banner Image

Environment

Command	Version
ruby -v
gem -v
bundle -v
bundle exec jekyll -v
bundle info jekyll-theme-chirpy

Additional context