Should images be separated from posts in MongoDb?
You feel like they should be separated because that makes perfect sense in the relational world.
But as you said in the NoSql world and in MongoDB in particular, you want to group like-items together.  I've not done extensive research on Mongo, but have spent some time with the online classes and I believe the answer is to store them together.  You probably realize there is no such thing as a join in Mongo and therefore if you wanted to get 100 rows and get their corresponding images, you'd have to get the IDs for the 100 rows and then get 100 rows by their identifier (object_id or whatever).  In other words, you have to do manual joins.