need to add custom logo with a image field

Hi.

I want to add a custom logo with MB settings page extension. Also want to update theme_mods customizer serialized data ..

When i add image field to settings page options saved to database like this:
s:11:"custom_logo";a:1:{i:0;s:3:"271";}}

But i need this image saves to database like this:
s:11:"custom_logo";i:271;}

( 271 is image atachment id)

How can i achive this ?

Thanks.