When we make Moments with only no picture tweets, we need a default cover media at Desktop(=includes Windows app) and iOS app. Android can set a cover media by accessing other apps like Google Photos.
Example at Android app:
Tap the picture icon,
then menu appeared.
Desktop and iOS app refer only local files, so we need a default cover media at Moment LIKE THIS.
2. Bookmark at PC. I wrote about "How to you Bookmark at your PC?" past.
Anyway, when will Bookmarks comes on Desktop? Until today(2018.5.25), if you want to use Bookmark at PC, should install Windows app or access to https://mobile.twitter.com. But, it doesn't make any sence. I think the number of user know can access Mobile ver. is minor.