I have a Windows Phone 8.0 witch can update the lock screen with count number and news updates. I have created a 38x38 icon with just white as color, but whatever I do it does not show up on the lock screen. The updates appear but the icon is displayed as an x.
You must manually edit WMAppManifest.xml and here is a property that specifies the path to your icon.
Take a look here: http://msdn.microsoft.com/en-us/library/windows/apps/jj207048%28v=vs.105%29.aspx
Working demo project: http://1drv.ms/1yxF7Qu