They all depict the ocean, and the writing across them is the opening verse of Bahá'u'lláh's Kitáb-i-Íqán (Book of Certitude):
العباد لن يصلوا الى شاطئ بحر العرفان الا بالانقطاع الصرف عن كل من في السموات والارض.
"No man shall attain the shores of the ocean of true understanding except he be detached from all that is in heaven and on earth."
Detachment is a concept that I've been pondering a lot lately. It seems to me that it is very important to be detached, not only in the quest for mystical knowledge, but also to really understand the divers situations life throws at us. It helps us to separate that which is true and meaningful from our vain imaginings and idle fancies and thus to make better decisions.