creating or updating a specific record based on foreign key

I have simply two table and models : photos and venues.
there is a polymorphic relationship between these two classes.
A venue has a special kind of photo which is a header photo, and there should exist at most one of those photos for each venue.(a venue can have none of it)

now How I can point that specif photo from from venues?
my photos table:

Schema::create('photos', function (Blueprint $table) {
        $table->increments('id');
        $table->string('path');
        $table->string('filename');
        $table->string('mime');
        $table->string('original_filename');   
        $table->integer('imageable_id');
        $table->string('imageable_type');
        $table->timestamps();
    });

I see two options, whether a foreign key on venues table pointing to a photos record or adding an attribute called is_header_photo to photos table.
for me, the first option seems right but I'm a bit confused how I'm I'm supposed to change image_id on venues table to point to the right id.
How can I update this field? and if you have a better solution I would be glad to be shared.

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