Save and Preview not working when customising new site

Hi,

I am very new to WordPress, and am having huge troubles customising this theme. I have changed the background image to one of my photos (correct size). I select Save and Preview. This works fine, however when I go to preview the site, it is the stock Boot Store images I am seeing. I have even tried editing the page from the Pages menu, but the same thing happens. What am I doing wrong?

Thanks!
Isla