It's been a while since I played trumpet, but from what I recall, resting your instrument's bell on the ground like that is a pretty big no-no.
Considering the trumpet is being held, I assume you meant the tuba. It is a no-no to do that without countering the main body weight of the instrument against something like a wall, or in this case she's sitting there holding against it.
Without that support on the opposite side of the bell, the weight can gradually wear on the bell and cause a bend, and the instrument is unbalanced so that it might tip over and be damaged even more than that. With support though, there aren't many better ways to put a tuba down short of a dedicated stand, but those take a lot of space and are very expensive. As long as the bell is even and the side is flush with a wall, you're fine.
