There is a corresponding movie to this album. I hope to report on clues within the movie soon.
The yellow submarine looks as though it is buried inside the hill that the Beatles are standing on. (read the Lyric Clues below to better understand)
This is the first occasion where we see an open palm above Paul McCartney's head. There are many different theories as to what this signifies. It's generally taken to mean either that the person under the open palm will soon die or has recently passed away. As you will see, this turns up quite a few times.
"...sky of blue, sea of green in our yellow submarine"
The yellow submarine represents a coffin, buried beneath a sea of green which represents the grass.
"... in the land of submarines"
In a cemetary - in the land of coffins.
'ALL YOU NEED IS LOVE'
"Yes he's dead... we loved you yeah, yeah, yeah."
If you listen closely to the end of the track, you can hear John Lennon sing the two lines mentioned above
'ONLY A NORTHERN SONG'
"When you are listening...you may think that the band is not quite right... the band is a little dark and out of key, you're correct, there's nobody there"
Paul played the bass guitar. Generally the bass player keeps the rhythm for the band. The band would not sound right if Paul was playing.