Anonymous Pick is right, these are usually used with a slotted head. Also note in your photograph that the posts are plastic and larger diameter than you normally. That's because they are usually used with nylon strings. They may work okay with steel strings, or the steel may cut into the plastic posts. You may be able to remove the plastic to get to the metal posts (not sure). Still, for the price, they are worth trying.
Hi, yep Toms dead right in time the steel strings will cause the plastic rollers to crack, I've seen it many many times. If you use them for steel strings and have to change them down the track, you have the problem of holes that are too large for the proper steel string posts.
Pretty nice set of classical guitar tuners there. Plus what everybody else said.