It's ackshyually quite simple as.
Posts with images take longer to post because of 8kun using a third-party image server (those ducks).
And, also, by the way timestamps are created many such that's spoopy comments appear on the board.
Now, when the post hits the front-end server (usually the load balancing rack) it gets a timestamp assigned.
No image, almost straight to the board it goes.
Images have to go through red tape like at the DMV and thus they arrive to the board a bit later.
With their hey I was here first dammit! timestamp.
>t. Now You Know