Assuming 2 things:
1. Your custom field is called: Image
2. The value of your custom field is something like: http://www.domain.com/images/myImage.jpg
Find this code in your template file:
<?php while (have_posts()) : the_post(); ?>
Immediately after add:
<?php $key="Image"; $postimage = get_post_meta($post->ID, $key, true); ?>
To display the image add the below code anywhere in The Loop where you want the image displayed:
<img src="<?php echo $postimage; ?>" />
Again, this is assuming your custom field is named "Image" and the value of that field is a fully qualified URL to an image file.
Hope that helps.