1. A method of determining and using motion vectors for a bi-predictive image block, the method performed by a moving picture coding system and comprising: determining, by the moving picture coding system, motion vectors of the bi-predictive image block based on a type of a reference picture of a co-located image block, the type being one of long-term and short-term, the type characterizing a temporal distance of the reference picture for the co-located image block with respect to the bi-predictive block, the determining step determining the motion vectors according to a first set of expressions if the reference picture for the co-located image block is the short-term type and the determining step determining the motion vectors according to a second set of expressions if the reference picture for the co-located image block is the long-term type, the second set of expressions being different than the first set of expressions; and decoding, by the moving picture coding system, the bi-predictive image block based on the determined motion vectors, wherein the second set of expressions includes a first expression which sets a first motion vector of the motion vectors equal to a motion vector of the co-located image block, and a second expression which sets a second motion vector of the motion vectors equal to zero.