Hello Rena,
Thanks for posting in!
When using the_post_thumbnail() or related functions, the ‘post-thumbnail’ image size is used by default, though a different size can be specified instead as needed.
You will need to choose which image size you want to display:
Post thumbnail sizes:
// Default WordPress
the_post_thumbnail( 'thumbnail' ); // Thumbnail (150 x 150 hard cropped)
the_post_thumbnail( 'medium' ); // Medium resolution (300 x 300 max height 300px)
the_post_thumbnail( 'medium_large' ); // Medium Large (added in WP 4.4) resolution (768 x 0 infinite height)
the_post_thumbnail( 'large' ); // Large resolution (1024 x 1024 max height 1024px)
the_post_thumbnail( 'full' ); // Full resolution (original size uploaded)
// With WooCommerce
the_post_thumbnail( 'shop_thumbnail' ); // Shop thumbnail (180 x 180 hard cropped)
the_post_thumbnail( 'shop_catalog' ); // Shop catalog (300 x 300 hard cropped)
the_post_thumbnail( 'shop_single' ); // Shop single (600 x 600 hard cropped)
// With X/Pro theme
the_post_thumbnail( 'entry' ); // Content Width
the_post_thumbnail( 'entry-cropped' ); // Content Width hard cropped
the_post_thumbnail( 'entry-fullwidth' ); // Full site content width
the_post_thumbnail( 'entry-cropped-fullwidth' ); // Full site content width hard cropped
Most probably, your code will be using like the_post_thumbnail('large') or the_post_thumbnail('full') to display the default WordPress image sizes. Feel free to change it with any of the available image sizes.
Hope this helps.