Can’t change homepage slider image

I want to change the image I’m using for the slider on my homepage (alastairgunn.com). however, when I click on the ‘Featured Slider’ section under ‘Appearance’ in ‘Vertex Theme Options’, there’s no function that allows me to do this. I’ve also tried going in through ‘Media – Library’ and editing the image I want to use, but no luck there, either.

I looked for advice on the forum, and thought it might be because I didn’t have the slider plug-in installed, so I installed the WordPress Slider plug-in (to no avail).

So I’m hoping someone out there can tell me why I have no option to change the slider image on my homepage, and what I can do to rectify the situation.

Thanks in advance
Alastair