For the displacement map the poly count has to be high (say 250 x 250 or may be more) . I feel the file has very less data for the kind of results you have in mind.

One option of displacement is in sculpt geometry tool under Polygon menu set.

Under attribute maps import the file name. Keep Import value as luminance. You need to play with the max displacement value. start with 200. Operation will be pull.

Mahesh