This is how to hide pictures and videos in HTC.Sometime during call you have to look into contacts but your phone doesn't brighten. Even if you shake your phone it not awake. This happen with me again and again. During call my HTC sleeps for a long time. Soon I realized a hidden feature in android.
There is a sensor in the earpiece of you cell phone. When you make or receive a call and put your cell phone near your ear the sensor activate because you blocked its waves and its screen goes to sleep. If you want to awake screen again you have to wave hand or thumb or anything in front of your cell phone's earpiece(it is that part of cell phone from which you listen the voice of the other caller). Your screen will awake and you can use your cell phone while attending a call.