Source

File: wp-includes/customize/class-wp-customize-header-image-control.php

public function print_header_image_template() {
		?>
		<script type="text/template" id="tmpl-header-choice">
			<# if (data.random) { #>
			<button type="button" class="button display-options random">
				<span class="dashicons dashicons-randomize dice"></span>
				<# if ( data.type === 'uploaded' ) { #>
					<?php _e( 'Randomize uploaded headers' ); ?>
				<# } else if ( data.type === 'default' ) { #>
					<?php _e( 'Randomize suggested headers' ); ?>
				<# } #>
			</button>

			<# } else { #>

			<button type="button" class="choice thumbnail"
				data-customize-image-value="{{{data.header.url}}}"
				data-customize-header-image-data="{{JSON.stringify(data.header)}}">
				<span class="screen-reader-text"><?php _e( 'Set image' ); ?></span>
				<img src="{{{data.header.thumbnail_url}}}" alt="{{{data.header.alt_text || data.header.description}}}">
			</button>

			<# if ( data.type === 'uploaded' ) { #>
				<button type="button" class="dashicons dashicons-no close"><span class="screen-reader-text"><?php _e( 'Remove image' ); ?></span></button>
			<# } #>

			<# } #>
		</script>

		<script type="text/template" id="tmpl-header-current">
			<# if (data.choice) { #>
				<# if (data.random) { #>

			<div class="placeholder">
				<span class="dashicons dashicons-randomize dice"></span>
				<# if ( data.type === 'uploaded' ) { #>
					<?php _e( 'Randomizing uploaded headers' ); ?>
				<# } else if ( data.type === 'default' ) { #>
					<?php _e( 'Randomizing suggested headers' ); ?>
				<# } #>
			</div>

				<# } else { #>

			<img src="{{{data.header.thumbnail_url}}}" alt="{{{data.header.alt_text || data.header.description}}}" />

				<# } #>
			<# } else { #>

			<div class="placeholder">
				<?php _e( 'No image set' ); ?>
			</div>

			<# } #>
		</script>
		<?php
	}