SQLFiddle-Demo
select products.id,
coalesce(t1.mid,t2.mid) as image_id
from products
left join (select min(id) mid,product_id
from images where `default`=1
group by product_id ) t1
on products.id=t1.product_id
left join (select min(id) mid,product_id
from images where `default`=0
group by product_id ) t2
on products.id=t2.product_id