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As mentioned above, this link can help you on your way:

You need to add thumnail sizes to your functions.php like:

the_post_thumbnail();                  // without parameter -> Thumbnail
the_post_thumbnail('thumbnail');       // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium');          // Medium resolution (default 300px x 300px max)
the_post_thumbnail('large');           // Large resolution (default 640px x 640px max)
the_post_thumbnail('full');            // Original image resolution (unmodified)
the_post_thumbnail( array(100,100) );  // Other resolutions

Then in your theme template within the loop where you call your image (for example loop.php), you need to change your image code into:

<?php if(has_post_thumbnail()) { the_post_thumbnail('thumbnail');  ?> // which will get you an image of 150x150