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;
}