One way to do this is to use feedback. Create a new picture that includes both the picture (in the exact same position as the original picture file) and text above it that says “respond now” - use photoshop or some other image editing program to create this picture. Create a new event and include this picture, then give this new event the input properties you want it to have once the participant sees the “respond now” prompt (e.g., ends after any response, no time limit).
Now select the original picture event (the one without the “respond now” text), and use the feedback tab. Select New and provide feedback if: There is no response within a given time. Select the “what to do” that appeared (present the same trial again) and delete it. Press the New button to create a new “what to do” and select “Present Event” and choose the new “respond now” picture you created above.
So what will happen is that if a participant does not respond within, let’s say 5000msec, it will automatically present the other image you developed that says “respond now.” To smooth the transition between these two events (so the participant doesn’t see a flicker on the screen when it advances to the “respond now” event), on the “respond now” event, click the presentation options tab and deselect the option “erase screen before presenting stimulus.” Now to the participant it will appear as though only the “respond now” text came up, rather than a flicker and then the picture/text.
All of this is assuming you’re using the most recent version of SL (4.0.7b). Good luck!