The iPhone accelerometer is, for all practical purposes, a good addition to a handheld device. However, sometimes it can be annoying, especially when trying to show off your favorite picture. The picture can flip between portrait and landscape modes over and over again as you move the phone or hand it off to a friend. The orientation change is enough to make you dizzy, but not if you use this tip.
Go to the Photos app and select a picture and while it is displayed, in either landscape or portrait mode, just tap and hold onto the picture with one finger for several seconds. If the "Copy" bubble appears you can let go and ignore it--it will vanish on its own. The picture is now locked into place and rotating the phone will not cause the orientation to change. To unlock it, you simply briefly tap the image with one finger again.
So far, this tip only works in the Photos app and not inside of other apps we tried. Also, it doesn't work when zoomed into a picture. It would be nice if the feature was global to iPhone OS, but unfortunately it isn't, according to our tests.