Force slideshow images to specific size?

I’m trying to control the size of the images shown in the slideshow. I have created a custom image size for the dimensions that I want, and I am using the shortcode that calls for that image size. However, the images being displayed are not using that size. How can I fix this?

The shortcode I am using is:
[gd_post_images type=’image’ ajax_load=’false’ slideshow=’true’ show_title=’true’ show_caption=’false’ animation=’slide’ controlnav=’1′ link_to=” show_logo=’false’ types=’post_images’ image_size=’cover-photo’ limit=’1′ limit_show=” cover=’n’ css_class=” ]

Image size created with this in functions.php:
add_image_size( ‘cover-photo’, 1200, 500, true );

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