Solution - 1

insert this code to functions.php of your theme.

	add_filter('attachment_fields_to_edit',  'medium_attachment_fields_to_edit', 0,  2);
	function medium_attachment_fields_to_edit($fields, $post){
	
		if (substr($post->post_mime_type,0,5) == 'image'){
		
			$fields['url']['html'] = str_replace("<button type='button' class='button urlfile'", "<button type='button' class='button urlfile' title='".esc_attr(array_shift( wp_get_attachment_image_src($post->ID, 'medium', false) ))."'>Medium File URL</button>\n<button type='button' class='button urlfile'", $fields['url']['html']);
		
		}
		 
		return $fields;
	}

Solution - 2

You can made it via CSS file. Look at HTML codes and find your links CLASS, for example: My site Our class is largelink, find it at your CSS and replase how you want. If you need more help please contact with me.

Solution - 3

Not sure what your use is for this but you could just insert a post thumbnail for the post: [[LINK href="http://wpengineer.com/the-ultimative-guide-for-the_post_thumbnail-in-wordpress-2-9/"]]http://wpengineer.com/the-ultimative-guide-for-the_post_thumbnail-in-wordpress-2-9/[[/LINK]] The issue is if you need this to change from large to medium to small size this may be difficult. Often times i have wordpress spit out both images and have jquery hide one or the other thumbnail based on the template. This could also allow you to (further down the line) to build something more complicated. Does that help?