The distribution of Frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species described, they are among the most diverse groups of vertebrates. Once adult Frogs reach maturity, they will assemble at a water source such as a pond or stream to breed. Many Frogs return to the bodies of water where they were born
A Male and Female common toad in amplexus. The black strands are eggs released into open water minutes after birth. Once at the breeding ground, male Frogs call to attract a mate, collectively becoming a chorus of Frogs.The male and female Frogs then undergo amplexus. This involves the male mounting the female and gripping her tightly. Fertilization is external: the egg and sperm meet outside of the body. The female releases her eggs, which the male Frog covers with a sperm solution. The eggs then swell and develop a protective coating. The eggs are typically brown or black, with a clear, gelatin-like covering. Most temperate species of Frogs reproduce between late autumn and early spring.
Source: en.wikipedia.org/wiki/Frog - 20.05.2010
It looks like the Frogs know, how perfect this environment fits them and as if they climbed into the blossom of the water lily for the sake of the picture.
But veinness is a human mannerism.