As for the frogs, I'm not sure why they're on the drums, but Vietnamese have a legend like this:
Once upon a time, the earth faced a long drought. Many living things withered and died. All animals were in thirst, but they didn't know what to do except lying on the ground, sticking their tongue out, waiting do die. Only the Toad is brave enough to set out a journey to heaven to demand the Gods Emperor to make rain. On the journey he met Crab, Tiger, Bee, Bear, Fox...Long story but in short, they all joined the Toad. When they arrived at heaven, the Emperor refused to meet them so he let the God of Thunder greet them. The God of Thunder scorned upon them and attacked them and long a messy battle occurred...but in short in the end, these animal scared the Emperor so he had to give in to their demands. The toad asked the emperor what if there's another drought in the future, should they come to Heaven again. The Emperor was terrified and said, "No you don't need to come up here. From now on, whenever you want rain, just grind your teeth and I'll hear it."
Since then, whenever toads grind their teeth, the sky will rain. So Vietnamese consider toads an omen of rain. They also have saying that, "The toad is the uncle of Heavenly God"
Frogs seem to have many symbols in ancient cultures too. It's not an exclusive trademark.http://en.wikipedia.org/wiki/Frogs_in_popular_cultureEgyptian mythology
To the Egyptians, the frog was a symbol of life and fertility, since millions of them were born after the annual inundation of the Nile, which brought fertility to the otherwise barren lands. Consequently, in Egyptian mythology, there began to be a frog-goddess, who represented fertility, named Heget (also Heqet, Heket), meaning frog. Heget was usually depicted as a frog, or a woman with a frog's head, or more rarely as a frog on the end of a phallus to explicitly indicate her association with fertility
The Ogdoad are the eight deities worshipped in Hermopolis. They were arranged in four male-female pairs, with the males associated with frogs, and the females with snakes
Hapy, was a deification of the annual flood of the Nile River, in Egyptian mythology, which deposited rich silt on the banks, allowing the Egyptians to grow crops. In Lower Egypt, he was adorned with papyrus plants, and attended by frogs, present in the region, and symbols of it.
The Biblical plague of frogs sent to curse ancient Egypt, like the nature of the other plagues, was intended to show the sovereignty of the God of Moses over the gods of Egypt.Ancient Greece and Rome
A frog being eaten by King Stork, an illustration by Milo Winter in a 1919 Aesop anthology
The Greeks and Romans associated frogs with fertility and harmony, and with licentiousness in association with Aphrodite.
The combat between the Frogs and the Mice (Batrachomyomachia) was a mock epic, commonly attributed to Homer.
The Frogs Who Desired a King is a fable, attributed to Aesop. The Frogs prayed to Zeus asking for a King. Zeus set up a log to be their monarch. The Frogs protested they wanted a fierce and terrible king, not a mere figurehead. So Zeus sent them a Stork to be their king. The new king hunted and devoured his subjects (as many human kings also do).
The Frogs is a comic play by Aristophanes. The choir of frogs sings the famous line: "Brekekekex koax koax."Ancient China
The frog represents the lunar yin, and the Frog spirit Ch'ing-Wa Sheng is associated with healing and good fortune in business, although a frog in a well is symbolic of a person lacking in understanding and vision.Indochina
To Vietnamese people, toad is the uncle of the Sky. According to a Vietnamese ancient story, whenever toads grind their teeth, it is going to rain.Scotland
In Scotland the frog is seen as an item of luck. Households often keep stone frogs in their gardens and they are often given as house warming presents. The associations are believed to date back to Pictish times.South America
Moche Toad, 200 A.D. Larco Museum Collection, Lima, Peru.
The Moche people of ancient Peru worshipped animals and often depicted toads in their art.East Lancashire
In East Lancashire the frog is also seen as an item of luck (as in the Scottish entry above). In this region ornamental frogs are often given as wedding presents, and in addition are seen as an aid to fertility and virility. This association is believed to date back to medieval times.