Encoder des images en base 64: http://base64online.org/encode/

Il est très performant de stocker des petites images utilisées fréquement dans la feuille de style.

.ImgCollapsed {
	width:20; height:20;
	text-decoration: none; 
	content: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABUAAAAQCAYAAAD52jQlAAAABGdBTUEAALGPC/xhBQAAAAlwSFlzAAAOwQAADsEBuJFr7QAAABh0RVh0U29mdHdhcmUAcGFpbnQubmV0IDQuMC42/Ixj3wAAAOlJREFUOE+tkCESwjAQRVcgEIhKRAUSgewhEByCQyCQOI7AETgCAoFAcgQEEoFAIBCI8FOS7k8mQzIDnXnT37y/O23FGPN3NIjULGJyntEgsgJbloz3oEp5RsNnyIYLaLhU4hkNOmR5gWVQzHhGQ/gmfngPhiWeSS219xm4uecrmIKvPruUzg7uzHJ099bjqgH7Nei1jhaklu7cmSVeWgH2+aW4/L+7g/jzGxB4v6ed7YIOncCT8qjEM6mlnk1QTHjQ507X7YLIwpUfYM6lEs9oQBGcwYQLpZ7RIDJmEWM9GKRcTPLwN4y8AfxNottsD8mSAAAAAElFTkSuQmCC");
}

usage en HTML:

<a href="#" onClick="alert('coucou');" title="Open"> <IMG CLASS="Collapsed"/> </a>

Exemple fiddle

On peut aussi stocker une image dans le javascript :

window.ImgCollapsed = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABwAAAALCAYAAABs3u23AAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAAYdEVYdFNvZnR3YXJlAHBhaW50Lm5ldCA0LjAuNvyMY98AAACtSURBVDhPY/j//z9dMZjYcOjK/9rZO3BikDyyJkowmAAZig9A5TE0k4PBBMjAP3/+wDEDUBjG/vb1A20s/PbtKxyDLATRHz+8/P/gwT0aWQj0CcgidHz31kWcFhobG/8HYWL5IAwmQAZ+/PgWjN++eQa26OXLx2Df3bx6mjYWvnh2A2w4CIMsvHbh2P9LZ/b/P3NyF22CFGTw/r0b/h/asRwDU91CuudD+uH/DADncY0ihCIzIAAAAABJRU5ErkJggg==";''