Move pictures to iCloud

I thought it was possible to move existing pictures from my iPhone storage to iCloud.

I’ve bought a 50gb iCloud plan, but when I activate the message sync option it only appears to sync new messages, not old.

is there any option to do so and how?

I’ve read that you have to sign in to the same Apple ID in all devices, I have a MacBook but it’s old and isn’t being used. Can it really be the problem?

I’ve 23GB of messages, tons of old messages I would like to keep somehow. Maybe iCloud ain’t the best solution? Any other ideas