loading images to show them offline

loading images to show them offline

I've made a web application that manage the offline state if the user
loose the connection. For that, I show information/error/... messages to
the user to inform about the offline state.
My problem is that some icons or images in the warning messages are not
shown because they are loaded when the user already has lost the
connection... The images are in the CSS not in the html :
.warning
{
background: #FFE680 url(/$App/IHM/Images/ico_warning.png) no-repeat 10px
center / 18px;
}
How can I preload the images/icons at the page construction to use them
offline ?
N.B : I can't use the HTML5 offline functionnality (manifest) because my
targeted users are on IE9 (html5 manifest is not supported) and for
personnal reason I can't use sprites...
Thanks for your help