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If you are looking how to insert a featured image inside a post, you can try the following solution.
Add the following code inside the functions.php of your theme:
[thumbnail size=medium align=right]
The side can be changed to small, medium and large values and/or align can be none, center, right or left.
Another way to control the thumbnail size with size=’array( 300, 200)’. It will look like this
[thumbnail size='array( 300, 200)']
Change the array value to a different number to achieve the result you need.