In order to replace default element images you need to modify servoy default css. See Customizing the Web Client for more info. The following examples are css classes that can be added in default css in order to modify default images.

.calendar img {
    background: url("http://www.servoy.com/developer/6xx_intro/resources/servoy_donut16x16.ico") repeat scroll 0 0 rgba(0, 0, 0, 0);
    height: 0;
    padding: 8px;
    width: 0;
}
.ui-state-default .ui-icon {
    background-image: url("http://www.servoy.com/developer/6xx_intro/resources/servoy_donut16x16.ico") !important;
    background-position: 0 center;
}
.ui-state-default .ui-icon {
    background-image: url("http://www.servoy.com/developer/6xx_intro/resources/servoy_donut16x16.ico") !important;
    background-position: 0 center;
}
.icon-panel img {
    background: url("http://www.servoy.com/developer/6xx_intro/resources/servoy_donut16x16.ico") repeat scroll 0 0 rgba(0, 0, 0, 0);
    height: 0;
    padding: 8px icon-panel !important;
    width: 0;
}

The url from css can also be embedded data url, for example:

background-image: url(data:image/gif;base64,R0lGODlhEAAQAMQAAORHHOVSKudfOulrSOp3WOyDZu6QdvCchPGolfO0o/XBs/fNwfjZ0frl3/zy7////wAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACH5BAkAABAALAAAAAAQABAAAAVVICSOZGlCQAosJ6mu7fiyZeK 
qNKToQGDsM8hBADgUXoGAiqhSvp5QAnQKGIgUhwFUYLCVDFCrKUE1lBavAViFIDlTImbKC5Gm2hB0SlBCBMQiB0UjIQA7);

See http://css-tricks.com/data-uris/ for more info on exact syntax.

addition_to_servoy_web_client_default.css