And maybe to save two related questions, why does Samantabhadra ride an elephant and Majursri ride a lion?
I was meditating with Kwan Yin statute and that was the main distraction, I had no idea why Kwan Yin is on a sea turtle.
Okay, I did my own research and here is what I've come up with so far. Interestingly, the artist is making a reference to this Pali suttra:
"Monks, suppose that this great earth were totally covered with water, and a man were to toss a yoke with a single hole there. A wind from the east would push it west, a wind from the west would push it east. A wind from the north would push it south, a wind from the south would push it north. And suppose a blind sea-turtle were there. It would come to the surface once every one hundred years. Now what do you think: would that blind sea-turtle, coming to the surface once every one hundred years, stick his neck into the yoke with a single hole?" "It would be a sheer coincidence, lord, that the blind sea-turtle, coming to the surface once every one hundred years, would stick his neck into the yoke with a single hole."
"It's likewise a sheer coincidence that one obtains the human state. It's likewise a sheer coincidence that a Tathagata, worthy & rightly self-awakened, arises in the world. It's likewise a sheer coincidence that a doctrine & discipline expounded by a Tathagata appears in the world. Now, this human state has been obtained. A Tathagata, worthy & rightly self-awakened, has arisen in the world. A doctrine & discipline expounded by a Tathagata appears in the world.
"Therefore your duty is the contemplation: 'This is stress...This is the origination of stress...This is the cessation of stress...This is the path of practice leading to the cessation of stress.'"
Other Bodhisattvas and Buddhas are often described as riding a particular animal.
Samantabhadra is explicitly described as riding an elephant in his sutra. Samantabhadra's elephant appears to be another awe inspiring flourish. (At least from my reading of the excerpt of Samantabhadra Meditation Sūtra in Wikipedia.
Wikipedia says of Manjusri and his lion: "Mañjuśrī is often depicted as riding on a blue lion, or sitting on the skin of a lion. This represents the use of wisdom to tame the mind, which is compared to riding or subduing a ferocious lion." Wikipedia doesn't explain why the lions, but elsewhere it's suggested that lions were sort a family symbol of the Sakyas, or it's just a reference to the pali sutra about "The lions Roar", ref: http://www.accesstoinsight.org/lib/authors/nanamoli/wheel390.html
The turtle/tortoise is a symbol of long life, health and medicine, as is the amrita nectar in the vase Kwan Yin is carrying. Kwan Yin herself symbolizes mercy, the moon, and the cooling waters of Dharma. I would think this is a prayer for long life, but also is a symbol of righteousness and compassionate living that will give you the causes for a long life to be able to practice Dharma, have better rebirths as well as ultimate enlightenment.
These are parts of later concepts and cultural development. To understand true Buddhism you have to try to develop understanding in the original Suttas and then put them to practice. This has little relevance to Buddhists than it being part of a cultural significance.
Higher understanding will come through your practice. Practice of the Dhamma and self realisation is the main aspect of Buddhism. So it would be best to set aside some of the question you might have for a while and than later revisit them through deeper knowledge you would have gained through meditation. As for the above these are cultural and later theological developments which have not weighting in the core practice and also some what difficult to answer in a definite way. So these kind of questions should be left aside for a while.