If you’re just trying to display one of four possible images per trial, you can use Feedback to end your trial after one image is displayed. Just go to the Event Editor window and click the Feedback tab. From here you can provide feedback Always, at the end of the event. Just set What to Do to Skip remaining events and present next trial. Oh, and don’t forget to randomize the events in the trail under the Trial Editor.
If you have a stimulus list, you can Expand Events that Use Lists. This option can be found under the Experiment menu in the menu bar. This will expand all events and trails that use the stimulus list. From there you can specify which images are associated with which trials.
Hope this helps.