Tutorial 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=">