Информация на их contributor help:
https://www.pixtastock.com/blog/keywording-advice/
Как видно примерах, там нет проблем с сочетаниями слов.
Major category keywords: e.g. portrait, scenery, plant, food
Minor category keywords: e.g. woman, Asian, townscape, green field, cherry blossom, cat, Japanese food, desserts
Detailed keywords(including proper noun): e.g. children, Japanese, African elephant, prunus yedoensis, Mt. Fuji, onion soup, apple pie
Explanatory keywords describing situation, movement, number, color, background: parent and child, smile, running, two people, blue, spring, night, outdoor, white background, New York