Problem when uploading images

In my custom theme I attempt to upload images and display them on a page template.

When I use <?php echo get_the_post_thumbnail($post->ID, ‘medium’); ?> or <?php echo get_the_post_thumbnail($post->ID, ‘thumbnail’); ?> it displays the image but not near the size I need.

When I use <?php echo get_the_post_thumbnail($post->ID, ‘large’); ?> or <?php echo get_the_post_thumbnail($post->ID, ‘full’); ?> no image is displayed.

The original image uploaded is not a small picture.

Any help is greatly appreciated.

I have also noticed that in my upload folder (which has full permissions) two versions of the image are saved: image-150×150.png and image-200×300.png. These are both much smaller than my original image.