"I don't think you can specify images as meta data."
Why so quick to decide that?
There a number of ways you can go about this. My suggestion is this bit of code:
<?php if(get_post_custom_values('img')) :
foreach(get_post_custom_values('img') as $value) {
echo '<img src="' . $value . '" />';
}
else :
echo '<img src="no-image.gif" />';
endif; ?>
Note this assumes use of 'img' for the custom field key. You may want to spiff up the img tags with height, width and title attributes. I also assume you're providing the full filename (and possibly path to image) as the field value, when you can use the script to take care of much of that. For example, if images reside in /images/, and all images will be GIFs, you could just use the name of the file without extension, and change the code to:
echo '<img src="/images/' . $value . '.gif" />';