I see someone else has already recommended using
the_post_thumbnail(), but I wanted to point out that you can use this function dynamically to create images of any size.
In your code, just use:
the_post_thumbnail( array(100, 150) );
This will display the post's thumbnail image at 100px wide by 150px high.
If your image isn't set as the post's thumbnail, you can use
wp_get_attachment_image() in a similar fashion:
echo wp_get_attachment_image( 1, array(100, 150) );
Just substitute the ID of the attached image for the
1 above, and the width/height you want. Be sure to use
echo because the function by default "gets" the image information and returns it ... you need to
echo it back to the browser.