How can i change the thumbnail photo display for my youtube video?

Unless you are eligible for YouTube partner program you have to be contained with the thumbnail that is assigned by default.

Each thumbnail is the frame at the exact middle point of the video, including the milliseconds. So if you upload a 4-minute long video (4:00:00), the thumbnail will be the frame that is at exactly 2:00:00. If you upload a 2:48:66 video, then the thumbnail will be the frame at 1:24:33. If you want to prove it to yourself, open a few of your uploaded videos in an editing program (e.g. Windows Movie Maker). Look at the video length, then divide it in half. The frame at that exact midpoint will be the thumbnail. YouTube has recently added a new feature to the “Edit My Video” option in the “My Videos” section. You still don’t have as much control over the method above but you can choose from three different images, taken from your video, to use as a thumbnail.