Is this sentence correct ? 我不知道会发我们的什么照片,所以就选择这个
I don't know to post which photo of us, so I chose this one

&quot;我不知道（应该）发我们的哪(一)张照片，所以我选(择)了这(一)张&quot;
Use 应该 instead of 会 when the person can decide what to do himself/herself.
Use 张 instead of 个 when referring to paper stuff.
which means 哪一(个/张/..） while what means 什么.
一 can be omitted in Chinese if no ambiguity.
Some examples:
I don't know what to do. 我不知道应该做什么.
I don't know what will happen. 我不知道会发生什么.
I don't know which to choose. 我不知道(应该)选哪一个.
一 张 纸/照片
一 个 东西/箱子
For accurate translation:
post: &quot;发布&quot;, post online; &quot;邮寄&quot;, post by mail