How to add background image to ionic side menu

You can define your custom CSS as follows:

.my-container {
    background-image:  url("http://www.planwallpaper.com/static/images/colorful-triangles-background_yB0qTG6.jpg");
    background-repeat: repeat;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    background-size: cover;
    height: 100%;
    position: absolute;
}
.transp .item-content {
    background-color: transparent !important;
    color: #fff;
}

and apply those classes to proper HTML items:

<ion-side-menu side="left">
    <ion-header-bar class="bar-dark">
        <h1 class="title">Menu</h1>
    </ion-header-bar>
    <ion-content class="my-container">
        <ion-list>
            <ion-item class="transp" menu-close href="#/app/dashboard">
                <i class="icon icon-dashboard" style="font-size:20px;margin-right:10px;"></i>Dashboard
            </ion-item> 
            <ion-item class="transp" menu-close href="#/app/dashboard">
                <i class="icon icon-dashboard" style="font-size:20px;margin-right:10px;"></i>Statistics
            </ion-item>             
        </ion-list>
    </ion-content>
</ion-side-menu>

Here is a codepen: http://codepen.io/beaver71/pen/WrqgNm