is the rear derailleur properly adjusted? Is the new chain the same length (same number of links) as the old? If you don't know how to adjust it, take the bike to your local bike shop or bike co-op and have it adjusted (the co-op will teach you how to do it). Worst case, replace the rear cassette; they wear out faster than the front chain rings. From the photos, the crank set looks fine.
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"Whenever I see an adult riding a bicycle, I know there is hope for mankind." (H. G. Wells)