public Style CaptionImageStyle { get; set; }

Public Property CaptionImageStyle As Style

<Window ...
        xmlns:dxdove="http://schemas.devexpress.com/winfx/2008/xaml/docking/visualelements"
        xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
        xmlns:dxdo="http://schemas.devexpress.com/winfx/2008/xaml/docking">
    <StackPanel>
        <dxdo:DockLayoutManager>
            <dxdo:LayoutGroup Caption="LayoutRoot">
                <dxdo:LayoutPanel Caption="Panel1" CaptionImage="{dx:SvgImageSource Uri='pack://application:,,,evExpress.Images.v20.2;component/SvgImages/Outlook Inspired/New.svg'}">
                    <dxdo:LayoutPanel.CaptionImageStyle>
                        <Style TargetType="dxdove:CaptionImage">
                            <Setter Property="Width" Value="16"/>
                            <Setter Property="Height" Value="16"/>
                            <Setter Property="Stretch" Value="Fill"/>
                        </Style>
                    </dxdo:LayoutPanel.CaptionImageStyle>
                </dxdo:LayoutPanel>
            </dxdo:LayoutGroup>
        </dxdo:DockLayoutManager>
    </StackPanel>
</Window>