Base64 Encode of 1x1px Transparent GIF

Just in case you need one. You can stretch it out to fill space as needed.

<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7">

Or   a black one:

<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAUEBAAAACwAAAAAAQABAAACAkQBADs=">