With the up/down separation of propagating wavefields using complex traces, (forward time extrapolated) source and (reverse time extrapolated) receiver wavefields are decomposed into up- and downgoing parts in acoustic reverse time migration (RTM). Together with the crosscorrelation imaging condition, four images (down-up, up-down, up-up, and down-down) are generated, which facilitate the analysis of artifacts and the imaging ability of the four images. Artifacts may exist in all of the decomposed images, but their positions and types are different. We have explained and illustrated the causes of artifacts in different images with sketches and numerical tests. The up-down image better images subsalt turned waves than does the down-up image; thus, decomposed images have advantages over a single composite image for target interpretation.