Pause and resume

Coding
5

Something like this (where images is a list of 30 pre-created image stimuli):

image_presses = [] # keep track of which images were paused

for trial in trials: # on each trial,
    for image in images: # draw 30 images
        for frame in range(30): # for 30 frames each (= 500 ms at 60 Hz)
            # draw the stimulus:
            image.draw()
            win.flip()

            # check whether to pause
            if event.getKeys('space'):
                paused = True # stop right here
                image_presses.append(frame) # store the current image number 
                # or could append(image.image) to store the actual image name.

                while paused:
                    if event.getKeys('space'):
                       paused = False
                    else:
                        # need to let the rest of system time to do stuff in this tight loop 
                        time.sleep(0.001) # 1 ms per iteration should do
                else:
                    # when exiting the while loop, also terminate the enclosing 500 ms 
                    # for loop and go on to the next image:
                    break

Note that as this code is based on drawing frames rather than keeping time, you can be paused indefinitely without issues.