The ClientImageLoaded client-side event doesn't fire for the first loaded image when DisplayAreaMode="Image".
The issue can be reproduced for the following configuration:
<telerik:RadImageGallery runat="server" Width="600px" Height="480px" DisplayAreaMode="Image">
<ClientSettings>
<ClientEvents OnImageLoaded="imageLoaded" />
</ClientSettings>
<ThumbnailsAreaSettings ThumbnailWidth="140px" Width="140px" Position="Left" ScrollOrientation="Vertical" />
<Items>
<telerik:ImageGalleryItem ImageUrl="images/Image1.png"/>
<telerik:ImageGalleryItem ImageUrl="images/Image2.png" />
<telerik:ImageGalleryItem ImageUrl="images/Image3.png" />
<telerik:ImageGalleryItem ImageUrl="images/Image4.png" />
</Items>
</telerik:RadImageGallery>
<script>
function imageLoaded(imageGallery, args) {
alert(args.get_image().src);
}
</script>