  Recent developments in astronomical radio telescopes opened new opportunities
in imaging and spectroscopy of solar radio bursts at sub-second timescales.
Imaging in narrow frequency bands has revealed temporal variations in the
positions and source sizes that do not fit into the standard picture of type
III solar radio bursts, and require a better understanding of radio-wave
transport. In this paper, we utilise 3D Monte Carlo ray-tracing simulations
that account for the anisotropic density turbulence in the inhomogeneous solar
corona to quantitatively explain the image dynamics at the fundamental (near
plasma frequency) and harmonic (double) plasma emissions observed at \sim
32~MHz. Comparing the simulations with observations, we find that anisotropic
scattering from an instantaneous emission point source can account for the
observed time profiles, centroid locations, and source sizes of the fundamental
component of type III radio bursts (generated where f_{pe} \approx 32~MHz). The
best agreement with observations is achieved when the ratio of the
perpendicular to the parallel component of the wave vector of anisotropic
density turbulence is around 0.25. Harmonic emission sources observed at the
same frequency (\sim 32~MHz, but generated where f_{pe} \approx 16~MHz) have
apparent sizes comparable to those produced by the fundamental emission, but
demonstrate a much slower temporal evolution. The simulations of radio-wave
propagation make it possible to quantitatively explain the variations of
apparent source sizes and positions at sub-second time-scales both for the
fundamental and harmonic emissions, and can be used as a diagnostic tool for
the plasma turbulence in the upper corona.
