How selectors with "sprite.png" works?

[Graphics Back Label]

Good morning,

 

I'm grappling with this " sprite.png " and I do not understand how it works ..

 

I understand it is an image -purpose , but I do not understand that logic functions with the selectors , as you type " doubles" .

 

Take a look:

On this page: http://www.backlabel.com/forward/204-cropped-leggings-performance-cotton.html

And even to this : http://www.backlabel.com/login?back=my-account#account-creation

 

You will notice that in the first , the selector of the size lacking the bullet markers ( and the right side ) , the same for the second , which, however, I have arranged with some " patch " , although in a mobile version continues to give the problem .

 

I ask just one more thing , why it is not left the original html ? If there a way , can you point it ? Because removing the background , I have left the box blank ..

 

Thanks so much,

Edited June 24, 2016 by Graphics Back Label (see edit history)