I looked at that theme, and it’s surprisingly easy to find. The code is in the theme-functions.php file, and it’s only at line 77 to 81, so right at the top of the file.
The bad news is that there’s no filters set up in the theme for image sizes, so what they’ve set is what’s set.
The good news is that you can set up a child theme, or a custom plugin, that can over-ride those settings. As long as you set up your action call with a priority of 11 or higher, you can remove sizes and add in your own.
Something (just typed in here, not tested) like this:
function change_image_sizes() {
add_image_size( 'gridbone-480w-480h-image', 480, 480, false );
// Etc for all image sizes as needed...
}
add_action ('after_setup_theme', 'chamge_image_sizes', 11);
After that, run a plugin that will re-do the thumbnails (I use Regenerate Thumbnails for this).
Just remember that you might get inconsistent results in your theme if the images are meant to be square, but end up being portrait or landscape, and this can very easily make your theme look weird.