Would the popup or the red text appear on the conponent content entry screen itself? or would it appear on the rendered page. I tried what you suggested and it seems to appear on the rendered page...Please help...
There's no way to make an image mandatory. I recommend that you make the default value of this image have big red letters that say "PLEASE SELECT IMAGE." Maybe it could be an animated .gif with blinking text.
That blank window with the word "false" is very strange. If you could open an issue with Tech Support and tell them what you did to get this window, we will look into it and see if it is a bug.