1. A system, comprising: one or more processors; and a memory coupled to the one or more processors and storing program instructions executable by the one or more processors to implement: accessing a retargeting matrix for an image, wherein the retargeting matrix is a data structure comprising an indication of a cost order for each of a plurality of seams in the image, wherein a seam represents a monotonic path of pixels from one edge of the image to the opposite edge of the image, wherein the pixels included in each seam are dependent on the respective energy values of the pixels, and wherein in the retargeting matrix the indication of the cost order for each seam is associated with each of the pixels of the seam; dividing the retargeting matrix into a plurality of sub-matrices, wherein each sub-matrix comprises the indication of the cost order for each seam of a subset of the plurality of seams in a respective portion of the image, and wherein each subset of the plurality of seams comprises two or more seams; identifying, in each of two or more of the plurality of sub-matrices, any pixels associated with an indication of a lowest-cost seam in the image; and removing or replicating the identified pixels in the image to produce a resized version of the image; wherein said identifying and said removing or replicating are performed independently and concurrently on the one or more processors for each of the two or more of the plurality of sub-matrices.