Preview won't show added picture
First you have to close all previously opened files (slides) in Preview. 

And also close all Preview windows. 

Check your Preview preferences settings:



Now start with your process and use Sort by "Name" in Preview.

To do that use Select all (Thumbnails) and right click on one of them, a new pop up will show the sort by.

In Preview File-Open, and open the slide you want to add.