Z-index conflict between menu & product images

Hello there everybody - hopefully somebody can help me!
I am not very knowledgeable with coding and all that tricky stuff, and somehow I have managed to create a problem with my top menu being visible over the top of my product popup images.

I have been trying to work this out for hours now and i just can't figure out where to change the code... even with firebugs help i can't work it out. So if you go to a product of mine ( daisuki.com.au/index.php/component/hikas...-phone-zipper-mascot ) and don't scroll down but click the image to bring up the pop up, you will see that the text from my top menu overlaps the image.

If anyone can help me figure out which file i need to change then I would really appreciate it, thank you so much