image lying over pop up box

Hi, my client has a 3rd party booking system on the website – a pop out calendar enables users to select a date – the problem is that an image is lying over this box – is there a way of making the image go under the calendar pop out? I’ve been told I need

I’ve been told I need to use css to play with the z-index property on the image and move it back so the picker sits over the top.

thanks,
sarah