Jeremy Herve on "[Plugin: Jetpack by WordPress.com] Photon destroyed "slides" featured images"

I had a look at the theme, and it seems to be using wp_get_attachment_image_src() to get the image, and then resize It. Unfortunately, that won't work well with Photon, unless you make a few changes.

Try the following:

1. In your theme's functions.php, add the following:
   

   function godsdead_jetpack_photon_image_downsize_array( $photon_args ) {
   	$photon_args['resize'] = $photon_args['fit'];
   	unset( $photon_args['fit'] );
   
   	return $photon_args;
   }

2. In your theme's header.php, locate the following line:

   <?php $image_url = wp_get_attachment_image_src(get_post_thumbnail_id(get_the_ID()), array(1500, 7000), false, ''); ?>

3. Delete that line, and replace it with the following:
   

   <?php
   	add_filter( 'jetpack_photon_image_downsize_array', 'godsdead_jetpack_photon_image_downsize_array' );
   	$image_url = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), array(1500, 7000), false, '' );
   	remove_filter( 'jetpack_photon_image_downsize_array', 'godsdead_jetpack_photon_image_downsize_array' );
   ?>

That should solve the issue.