How can I get one image-of a pair-to display depending on condition chosen?

Hi. I'm trying to create a task using Psychtoolbox. Basically, I'll have two images displayed, under them will be two questions/conditions. The subject will choose one of these conditions. The picture that was on the same side as that condition must then be displayed in the next "frame" (not sure what exact word is). How can I tell PTB/Matlab to do this? Thanks!