class One(models.Model):
...
    def img(self):
        return Two.objects.filter(rel=self)
class TwoImg(models.Model):
    rel = models.ForeignKey(One, on_delete=models.CASCADE)

One.objects.get(pk=1).twoimg_set.exists()

One.objects.get(pk=1).twoimg_set.exists()

{% fon n in one %}
{{n.img}}
{% endfor %}

False
True
True
False
...

{% fon n in one %}
    {% if n.img %}
        {{ n.img }}
    {% endif %}
{% endfor %}

{% fon n in one|slice:":10" %}
    {% if n.img %}
        {{ n.img }}
    {% endif %}
{% endfor %}

a0 = One.active_objects.select_related().all().order_by('-since')
a1 = a0.values_list('rel', flat=True).distinct()
b = a0.filter(pk__in=list(a1))[:10]

{% fon n in one %}
    {% if n.twoimg_set.exists %}
        {{ n.img }}
    {% endif %}
{% endfor %}

from django.db.models import Count
one_objects = One.objects.filter().annotate(Count('twoimg')).filter(twoimg__count__gt=0)

{% fon n in one_objects|slice:":10" %}
      {{ n.img }}
{% endfor %}