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>