Make sure that they are at the right resolution (.dpi) that they can print them; I think at least 300 dpi is OK but check first. Make sure the picture is in focus and sort out red eye etc.
In particular certain compositions seem to work better than others. Magazines don't want pictures with big empty gaps in them; they want something close up ideally. Have a look at the magazine you're aiming for and look at the pictures not only for content but also for layout.