Upload an image in customizer then select thumbnail

Hi,

I’m developing a theme that allows users to upload images in the customizer.

I have been outputting the image url using get_theme_mod('image_setting'); but I want to output the url of thumbnail version of this image.
All I seem to have access to is the url taken from get_theme_mod().

Is there a solution of gathering the images different urls based on the original url outputted by get_theme_mod ?

Thanks
Steven