This image is taken from the Public Health Image Library (PHIL), a division of the CDC. The description of the image specifically states that it is in the public domain: "This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image." It is credited to John Willson at the Savannah River Ecology Laboratory (SREL). I downloaded the high-resolution TIFF from the website and re-saved it in the PNG format (compression level 9) using GIMP. This reduced the file size by about half while resulting in no loss of visual information.
Source: https://commons.wikimedia.org/wiki/File:Nerodia_fasciata_CDC.png

The banded water snake or southern water snake (Nerodia fasciata) is a species of mostly aquatic, nonvenomous, colubrid snake endemic to the central and southeastern United States. It is found from Indiana, south to Louisiana and east to Florida. Order: Squamata, Suborder: Serpentes, Family: Colubridae, Subfamily: Natricinae, Genus: Nerodia, Species: Nerodia fasciata (Linnaeus, 1766).
Subspecies:
Nerodia fasciata confluens (Blanchard, 1923)
Nerodia fasciata fasciata (Linnaeus, 1766)
Nerodia fasciata pictiventris (Cope, 1895)
Synonyms:
Coluber fasciatus Linnaeus, 1766
Tropidonotus fasciatus — H. Boie, 1827
Nerodia fasciata — Baird & Girard, 1853
Natrix fasciata — Cope, 1888
Natrix sipedon fasciata — Stejneger & Barbour, 1917
Natrix fasciata — Conant, 1975
Nerodia fasciata — H.M. Smith & Brodie, 1982