"-shima" is just 島 'island'. See also Ōshima, Mishima, Fujishima, Furushima, Hiroshima, and even Iwo Jima (硫黄島, "Shima" is sometimes pronounced "Jima" in Japanese for some reason).

Note that Fukushima isn't a "true" island, though some think it might come from a block of land isolated by rivers.