Designer files do not go in assets as 'files' of ides goes.
You can have different image files here to put in imageview, than those you will need to run the app.
These files are only in .bal (designer) for layout display.
What I see.... If I add a image in Designer's Files, a file appears in both tabs - in Designer and in Files Manager.
.bil file includes the links only (.bil is 3K only, meanwhile the image size is 100K). So no reason to think that Designer's files are not in Assets.