Thumb.jpg not shown in Admin Plugin Page Media

I have a modular page with a set of pictures (01.jpg - 12.jpg). When I add a set of thumbnails (01.jpg.thumb.jpg - 12.jpg.thumb.jpg) and save the page, the thumbnails aren’t shown in Page Media yet they are working with display(‘thumbnail’). When I want to change one of the pictures (01.jpg) I would need to delete the according thumbnail (01.jpg.thumb.jpg) manually via ftp because I can’t see it in Page Media and it also isn’t deleted automatically when I delete the “original” picture (01.jpg). Is this a bug, or am I misunderstanding something here?

Also is there any drawback in using cropResize on the original images instead of providing thumbnails?