Hello, Dear Ana!

1.If you want to change breadcrumbs image globally, please click WP Dashboard => Appearance => Customize => Additional CSS and paste this code here:

.entry-header-wrapper {    }

Then save changes and check results at front-end. Use your custom path-link for image.

If you want to assign the different breadcrumbs images for, please obtain the id of this page via the plugin: https://wordpress.org/plugins/reveal-ids-for-wp-admin-25/

Then add in Additional CSS this code:

.page-id-1175 .page_title {    }

where .page-id-53 is the necessary id of your custom page.

Save changes and check results at front-end. Using this method you may assign a custom image for every page.

2. If you want to set up the form more wider, please click WP Dashboard => Appearance => Customize => Additional Css and put the following code: 

.form-control {
    display: block;
    width: 100%;
}

Best Regards!