Featured images and slider images cropped, not resized

My theme, Unicorn by Ridwanreedwan has a slider on the front page of my site (http://theprimpingpress.com/). The slider just takes the featured image of my blog posts and crops them, rather than resizing them to fit the space. If you look at the second image that slides by, you’ll see it’s awkwardly cropped.

Again, the same thing happens when you go to the post. But when you click on the photo, it pops up in all of it’s glory. I can’t have it cropped! Is there a way to have it shrink to fit the area, rather than cropping it?

I think these images are called the thumbnails — correct? If so, here’s the code in my theme:

<?php

// Post Thumbnails
if ( function_exists( 'add_theme_support' ) ) { add_theme_support( 'post-thumbnails', array( 'post', 'page' ) ); }
add_image_size( 'full-slider', 956, 450, true );
add_image_size( 'small-slider', 628, 356, true );
add_image_size( 'big-magz', 430, 244, true );
add_image_size( 'small-magz', 80, 60, true );
add_image_size( 'carousel', 199, 129, true );
add_image_size( 'related-post', 280, 161, true );
add_image_size( 'content-magz', 628, 250, true );
?>

I also downloaded the plugin “Simple Image Sizes” and played around with it (most likely totally incorrectly!) and managed to resize the slider size to fit my photo perfectly. The downside? It didn’t actually SHRINK my photo, but just stick a huge photo in the middle of a slider, messing up my site royally.

And now, I give up. Can someone please help? Or is the only solution to somehow make every photo the exact 628×356 dimensions of that space? (Which would suck).