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<?php
$attachment_id = get_field('field_name');
$size = "full"; // (thumbnail, medium, large, full or custom size)
$image = wp_get_attachment_image_src( $attachment_id, $size );
// url = $image[0];
// width = $image[1];
// height = $image[2];
?>

<img class="image-class" alt="" src="<?php echo $image[0]; ?>" />

<!--
More Info Here:
http://support.advancedcustomfields.com/forums/topic/custom-image-sizes/
-->

<?php
add_image_size( 'category-thumb', 300 ); // 300 pixels wide (and unlimited height)
add_image_size( 'homepage-thumb', 220, 180, true ); // (cropped)

// May need to use https://wordpress.org/plugins/regenerate-thumbnails/ after
?>


<!--  FROM WORDPRESS -->
<?php add_image_size( $name, $width, $height, $crop ); ?>

<!--

$name
(string) (required) The new image size name.
Default: None

$width
(int) (optional) The post thumbnail width in pixels. Set to 9999 to resize based on $height
Default: 0

$height
(int) (optional) The post thumbnail height in pixels.
Default: 0

$crop
(boolean/array) (optional) Whether to crop images to specified height and width or resize Difference between soft and hard crop

false - Soft proportional crop mode.
true - Hard crop mode.
array - Specify positioning of the crop area (x_crop_position, y_crop_position).

Default: false

-->