Preferred WP/ACF image usage

In ACF, try to never use the_sub_field() on an image and always have ACF return an image array, especially for larger images. Calling the_sub_field() directly will bypass WordPress’s image sizing code and should only be used if you have an explicit size that designers are cropping to in Photoshop.

To use a custom size, you need to register it in function.php, usually in after_setup_theme, via the add_image_size() function.

add_image_size( 'your-custom-image-size-name', width, height );

For instance:

add_image_size( 'home-featured-service', 900, 900 );

Once you do this you can have WordPress generate your src, srcset and sizes attributes for you:

//Image stored in ACF as an array instead of image
$image = get_sub_field( 'image' );

//Get the default image SRC
$img_src = wp_get_attachment_image_url( $image[ 'ID' ], 'home-featured-service' );

//Get additional srcsets
$img_srcset = wp_get_attachment_image_srcset( $image[ 'ID' ], 'home-featured-service' );

//Get the sizes attribute
$img_sizes = wp_get_attachment_image_sizes( $image[ 'ID' ], 'home-featured-service' );

And then write the tag:

    src="<?php echo esc_url( $img_src ); ?>"
    srcset="<?php echo esc_attr( $img_srcset ); ?>"
    sizes="<?php echo esc_attr( $img_sizes ); ?>"
    alt="" />