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Post and pages featured image settings can be found in two places
1) Global settings:
A) for posts (apply to all posts)are in:
Main options > Settings > Post Specifics> Featured image section.
or Customizing ▸ Images > Post Specific
B) for pages (all pages) are in:
Main options > Settings > Content Areas > Featured image section.
or Customizing ▸ Images > Content
2) Local settings:
A) specific post, in the post editing page > Xtreme Option for this post > Post Blog Archive options TAB > Featured image drop down list
B) specific page, in the Page editing page > Xtreme options for this page >Layout TAB > Featured Image > How to display featured image
If you need more help, we need a link to the page where the featured image is replacing the header image so we can be sure of your specific situation