WP Login and Dashboard Customization

I need some help here, I'm coding a plugin to change the wordpress interface and I was having issues with the Images paths.

For example:

In my plugin dir I putted the .PHP file with the code and css, and an images folder, to contain all images, the thing is that the only way I managed to call the images is by the complete URL and I wanted to use relative or absolute URL for the plugin.

I call the css like this.

.login h1 a {
-webkit-background-size: 150px;
background-size: 150px;
background-position: center top;
background-repeat: no-repeat;
height: 150px;
margin: 0 auto 25px;
padding: 0;
width: 220px;
text-indent: -9999px;
outline: none;
overflow: hidden;
display: block;

I wanted to use a relative URL so wen a client purchases the plugin I can provide him with the images on the folder and the plugin itself calls the images on his server and not mine.

Thanks for the help