How do I get a single image for a post with multiple images?

[kindo]

I am working on a web app in Python (using the Flask framework). I have a post and photo table connected via ForeignKey, my problems:
1. how can i return an empty image if image is empty, and if not empty
2. how do i select just one image from the list of images.

Here is the model:

class Photos(db.Model):

    __tablename__ = 'photos'

    id = db.Column(db.Integer, primary_key = True)
    postid = db.Column(db.Integer, db.ForeignKey('post.id'))
    filename = db.Column(db.String)

class Posts(db.Model):

    __tablename__ = 'post'

    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(30))
    photos = db.relationship('Photos', backref='user', lazy='dynamic')
    created_at = db.Column(db.DateTime, default=db.func.now())
    status = db.Column(db.SmallInteger, default=1)


    def thumb(self):
        """
        Get post photos
        """
        if self.photos is None:
            return '/static/images/empty.png'
        else:
            for p in self.photos:
                return '/static/images/%s' % (p.filename)

Usage(the View):

For single image (doesn't work):
{{post.thumb()}}

Loop through images (this works):

{% for photo in post.photos %}
    <img src="/static/images/{{photo.filename}}"/>
{% endfor %}