This is simply because your browser in Android is already linked to Images gallery as a default app servicing image uploads. Android asks about your preference the first time you try to use this routine and then remembers your choice. 

UNA app would be treated as a separate app, not a website within browser, so Android needs to ask what do you want to use as a file service for image upload for this app - it should only do it once. In most cases it won't do it at all, since most people only have the default Images app to supply image uploads.