可以用video中的html5poster属性
poster="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"
data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7为最小的base64图片并且透明标签：image,base64,yH5BAEAAAAALAAAAAABAAEAAAIBRAA7,gif,安卓七,webview,data,图标 From： https://www.cnblogs.com/Old-vegetable-chicken/p/17348997.html
也可以选择黑色：
data:image/gif;base64,R0lGODlhAQABAIAAAAUEBAAAACwAAAAAAQABAAACAkQBADs=