Why does my iPhone only allow me to send picture messages to other iPhone users?

How can I fix this? It's the 3gs

It's with T-Mobile by the way. Had it unlocked

The way that comes to mind, go to settings messages and make sure that iMessage and mms are both on. If iMessage is in and mms isn't then it could cause this problem as iMessages only go to iOS products with apple id, such as iPhone. If mms ( photo and video messages) is turned off, you wouldn't be able to get them. Data needs to be on to send them both as well.