1. A method of estimating lateral positioning uncertainties of a seismic image, the method comprising: at a computer system having one or more processors and memory storing programs to be executed by the one or more processors: receiving a velocity model, the velocity model including a plurality of base velocity values used for generating the seismic image, each base velocity value having a low limit and a high limit; deriving a plurality of lateral velocity gradient uncertainties from the velocity model; generating multiple lateral velocity gradient profiles, each lateral velocity gradient profile including a random realization of the plurality of lateral velocity gradient uncertainties; calculating perturbation raypaths originating from a surface point of the seismic image based on the velocity model and the multiple lateral velocity gradient profiles; and estimating a lateral positioning uncertainty for a target location at a predefined depth of the seismic image based on lateral distributions of the perturbation raypaths at the predefined depth.