Multiple Featured Image

Hi guys

I’m currently pretty new with all this stuff and I wanted to set different
“Featured Images” up for a Post.
The problem is that I’m using different images for different pages.
For Example: Slider has image Size 600×400 and Single Post 150×150

The problem is that I don’t have any idea how I could solve it.
I already googled but couldn’t find anything which would be helpful and I’m now searching for help here.

The current code I’ve for the image is:

<?php if ( has_post_thumbnail() or  get_post_meta($post->ID, 'inner-post-image')) { ?>

            <div class="img_in">

              <?php if( get_post_meta($post->ID, 'inner-post-image')){?> <img src="<?php $img = get_post_meta($post->ID, 'inner-post-image'); echo $img[0];  ?>" /> <?php  }else{ the_post_thumbnail();} ?>

              <?php }else{?>

               <div class="img_in">

              <img src="<?php echo get_template_directory_uri() ; ?>/images/nophoto.jpg">

              <?php } ?>

            </div>

          </div>

I hope it’s right so far for using the normal Featured Image.
Do you have a solution for it? I wanted to use the plugin “Multiple Featured Images” but didn’t really understand how and which lanes to replace.
I’m really sorry for being new and if it was posted a lot already, but I’m really trying hard to learn it somehow.

Thanks a lot