Yukata is more of a thin fabric, and often worn in the Summer time-especially during festivals.
Kimonos are more expensive and are often worn during a really formal event (e.g. someone's wedding, Coming of age day, etc...) Though, children will wear the kimono-well, the girl at least, wears it on Shichi-Go-San, Girl's Day, etc....
The first picture is a kimono, but is referred to as a Furisode-distinguishable by its long sleeves. Furisode is worn by the younger girl or unmarried woman. The pattern of the kimono in the first picture also has typical Furisode pattern.
My favorite to wear is the furisode, because of the elaborate pattern. Yukatas are typically more plain pattern-style wise.