The magnetic domain pattern in (Ga,Mn)As layers with perpendicular magnetic easy-axis is studied by Kerr microscopy at various temperatures. We investigate the effect of post-growth annealing. As-grown samples exhibit a strongly anisotropic domain growth with dendritic-like expansion of domains along the < 110 > directions. For annealed samples, the density of nucleation centers is decreased and domain wall propagation is more isotropic. Two kinds of pinning centers are observed in annealed samples: line-like defects that strongly hinder domain wall propagation and point-like defects with a density of a few 10(5) cm(-2).