I see two possibilities here:
- The second image will need to be transparent to be drawn round over top of the square.
- The second image will need to be a composite image of both the circle and the square.
I would recommend the second approach, since you’re running on Windows. It’s been a long time since I’ve played with transparent images in SuperLab, but I had issues on Windows, and I seem to recall disabling transparency in the Windows version of SuperLab.
If you want the time between the first and second stimulus to be constant, you will need to set the first event to end after a time limit. If you want a response during the first event to cause the trial to end, you need to set up a feedback to that event that will cause the remaining events in the trial to be skipped.