Removing main photo in a slider using leo bootstrap

[IsaG]

I am creating sliders in leo bootstrap theme and all has gone well (thanks mostly to copying the format created by my web developer, so with little knowledge on my part).

But there is one slider giving me trouble: I wanted to change the main photo on one of the sliders but couldn´t see how to do this: sometimes there was a tiny frame that appeared "change photo" or something similar but it didn´t always appear.  There didn´t seem to be a delete photo button anywhere.  So I uploaded a new photo on top of the old photo and now the old slider, I guess "hidden" behind the new one, appears on the site just before the "new" one. 

Is there any way to get rid of the main photos so that I can start again?  I want to use this particular slider again as I have also prepared the same slider in 2 other languages under the same theme and so don´t want to have to do the whole lot again if there is a faster and easier way to get rid of the old photo.

Thanks.