why OH WHY does everybody insist on making custom fields for everything .
is there a special reason that you would want the use of custom fields ?
If so, please explain exactly what you want to achieve .
If not – a much better approach (and more fool-proof for non-computer-savvy-clients) would be to use the_post_thumbnail(); – like you stated in your question already, with Additional image sizes in the theme .
I do not want to duplicate my answers – so you just as well read here :
Display thumbnail from custom field
Basically you will use
<a href="https://wordpress.stackexchange.com/questions/38886/<?php the_permalink(); ?>"><img src="the_post_thumbnail('your_size_name');" /></a>