1.Did you find those pictures were taken in Beijing?
改正：去掉were.
该句谓语为find,所以taken in Beijing做后置定语,不能用were taken in Beijing这种谓语结构.
2.I'll do all what I can to help you.
改正：what改成that,或者去掉what.
all是不定代词做先行词,that代替all并做can后面省略掉的do的宾语；如果去掉all,则what I can (do)为宾语从句.
3.The students who has finished the exericses may leave the classroom now.
改正：has改为have.
who相当于先行词the students,当然其谓语have不能用三单形式.