Yeah what he said. I'll add that if the pics in this timeline gallery are pulled from other folders, for example on my device I don't have anything called a timeline gallery but I do have an all pics folder. You'll want to take care that you don't delete them from there because it deletes the source not just the all pics folder. What you're probably more interested in is preventing them from showing all together and there are several ways to do this. There are apps that hide photos, probably the easiest way to go. Or you can create a folder to store pics and vids in and create a no media file to put in that folder. A no media file is a simple text file that has one or two lines in it that will cause your gallery to over look it when gathering files. The drawback to this is you'll have to go to the folder to view any of the files.