How Do I Make It Use My Index?

Hello!

I have two tables

users and pictures.

table users have a clustered (PK) index on userid
table pictures have a clustered (PK) index on userid

when I do this query:

"select userid from pictures where userid=123"

then It will do a clustered index seek

But If I do any of those:

"select t2.userid from users t1 left join t2 on t1.userid = t2.userid"
or
"select (select userid from pictures where usedid = t1.userid) from users t1"

It will do a clustered index scan.

How can I force it to seek my index instead of scan?

Thanks!