Under iOS Basic, a default splash screen will be displayed while your game loads, oriented according to the Default Screen Orientation option in the Player Settings.
Users with an iOS Pro license can use any texture in the project as a splash screen. The size of the texture depends on the target device (320x480 pixels for 1–3rd gen devices, 1024x768 for iPad mini/iPad 1st/2nd gen, 2048x1536 for iPad 3th/4th gen, 640x960 for 4th gen iPhone / iPod devices and 640x1136 for 5th gen devices) and supplied textures will be scaled to fit if necessary. You can set the splash screen textures using the iOS Player Settings.