The standard iPhone photo slideshow settings employ a three-second period of time for each photo to rest on the screen and a dissolve transition between each picture. Three seconds is usually considered the base standard for people being able to appreciate an image fully. Four or five seconds may also be acceptable, but any less will not give the image enough screen time to get the picture fully interpreted. You also have the option to keep the image on screen for ten or twenty seconds, which is too long for almost any practical situation.
The dissolve transition is the most common video transition in all media and therefore is the least obvious. It is the only available transition that will not draw attention to itself, but if you would like to then you can go ahead with the wipe. The wipe is good for comic effect, so make sure that the photo slideshow is conducive to this type of presentation.