Welcome to the Forum! Lots of great people here!
To my knowledge there isn't a way to change it with the default gallery app you may be looking for a replacement gallery app to get rid of the preview. Unfortunately I haven't tried any others, aside from JustPictures which I've used for years to connect with my picasa account. JustPictures shows a folder list, but has a thumbnail of a photo from each folder... maybe someone else can chime in with a good replacement option!