This is very likely some species of **willow** tree (genus **_Salix_** ).
Without a better photo it will be difficult to ID this to species. To complicate matters, willows are very cross-compatible, and numerous hybrids occur, both naturally and in cultivation.
Since it appears your picture is from a garden or park, a fair guess would be the common ornamental tree:
**Weeping willow** (commonly _Salix × sepulcralis_)
 from China and white willow ( _Salix alba_) from Europe