The main challenge and deficiency with this and other services like this is that they assume some content type to be foreground - here it is likely "portrait" style people - it's hard to say as this information isn't included in the UX. Does it handle other foregrounds, such as cars? Who knows.

A better solution is to have controls, at least in optional manner, which the user can use to scribble examples of the foreground(s) and backgrounds.