You can create a category, "feature". Make a note of its ID, you'll need this in a minute.
Then, in your sidebar, put something like this (your template may be slightly different):
<?php query_posts('cat=X&showposts=1'); ?>
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<h2" id="post-<?php the_ID(); ?>"><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h2>
<img src="/your/path/to/image/<?php $values = get_post_custom_values("Image"); echo $values; ?>" alt="" class="magcover" /><?php the_excerpt(__()); ?>
<?php endwhile; else: ?>
<?php endif; ?>
This tells WP to look for only posts in the category of "Feature" (whatever that ID is) and only show the latest post from that category.
You'll also have to use a custom field where you put the magazine image and you'll need to further revise that sidebar code to pull that image in. Read more about that here:
And particularly to do what you want, which is to add images:
<img src="/your/path/to/image/<?php $values = get_post_custom_values("Image"); echo $values; ?>" alt="" class="magcover" />
See that I've placed that code above in the sidebar code that you'll need to use. I separated it out just so you could see how it works a bit better. All you need to do when you post is declare your custom field to be Image and upload your magazine cover via the posting page. Substitute the actual path to your image uploads that you normally use, and you can use a styling hook, in this case the class "magcover" -- or whatever you want to call that style class -- to style the appearance of that image. Put that path in the "variable" box in the custom field and WP will insert it into the sidebar code.