Check out snowdrift crabapple. They are one of the most popular white varieties, and the size and shape are right for that picture. I've planted lots of them.
http://edis.ifas.ufl.edu/ST395
If the flowers (and fruit!) are divided into five parts, you will have something in the apple (Rosacea) family. Close examination of the flower and other parts will nail it down.
Pistil or ovary 1 in each flower: it's a prunus
Thorns present: Craetegus (Hawthorn)
Ovary 2-5 cells, fruit 2-5 celled, each carpel 1 or 2 seeds, Stamens 15-50: Pyrus (Pears, crabapples, apples)
Most crabapples are pink-red. I'll bet it's a snowdrift.
There are LOTS of other white crabapples:
Scientific Name Common Name Height (ft.) Zone
Malus 'Amberina' flowering crabapple 7-10 4-8
Malus 'Amerspirzam' AMERICAN SPIRIT crabapple 15-18 4-8
Malus 'Beverly' flowering crabapple 15-25 4-8
Malus 'Callaway' crabapple 15-25 5-8
Malus 'Camzam' CAMELOT flowering crabapple 8-10 4-8
Malus 'Donald Wyman' flowering crabapple 15-20 4-8
Malus floribunda Japanese crabapple 15-20 4-8
Malus 'Hozam' HOLIDAY GOLD flowering crabapple 15-18 5-8
Malus 'Jewelberry' flowering crabapple 7-8 4-8
Malus 'Katherine' flowering crabapple 15-20 4-8
Malus sargentii 'Tina' Sargent crabapple 2-4 4-8
Malus x scheideckeri 'Red Jade' weeping crabapple 12-15 4-8
Malus 'Snowdrift' flowering crabapple 15-20 4-8
Malus 'Sutyzam' SUGAR TYME flowering crabapple 14-18 4-8
Malus x zumi var. calocarpa redbud crabapple 12-20 4-8
I hope this is helpful.