The feature of jsitemap uses the standard Joomla API for custom meta tags, thus the itemprop="image" addition is not supported. You should switch to a custom tag API instead.
To do this you should customize the source code of the plugin file:
root/plugins/system/jmap/jmap.php
at the line 323, changing from:
Code: |
$document->setMetaData('og:image', $imageLink, 'property');
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to
Code: |
$document->addCustomTag('<meta property="og:image" itemprop="image" content="' . $imageLink . '" /> ');
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The same is valid for the addition of og:type, you can add below the line:
Code: |
$document->setMetaData('og:type', 'article');
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By the way if you need advanced management of Open Graph tags, you may consider to switch to another specific and more advanced plugin, obviously this is not a key feature of jsitemap.
Best regards