"Icon" is a property on the main <sap:ActivityDesigner> element of the designer and it's of type "DrawingBrush". 

    If you want to set an image (BMP, JPG...) as the Icon, just add this XAML exactly after the starting element of the designer: 代码

<sap:ActivityDesigner.Icon> 

        <DrawingBrush>
            <DrawingBrush.Drawing>
                <ImageDrawing>
                    <ImageDrawing.Rect>
                        <Rect Location="0,0" Size="16,16" ></Rect>
                    </ImageDrawing.Rect>
                    <ImageDrawing.ImageSource>
                        <BitmapImage UriSource="D:\Public\Foto\IMG_2560.JPG" ></BitmapImage>
                    </ImageDrawing.ImageSource>
                </ImageDrawing>
            </DrawingBrush.Drawing>
        </DrawingBrush>
    </sap:ActivityDesigner.Icon>

     Obviously, change the UriSource to match your Icon/Bitmap. If you want to apply a WPF DrawingBrush, design the Icon with Expression Designer and copy/paste XAML/ResourceDicrionary and use it.
    If you want to set a vectorial drawing, like I do:

代码

<sap:ActivityDesigner.Icon>
        <DrawingBrush>
                <DrawingBrush.Drawing>
                    <DrawingGroup>
                        <DrawingGroup.Children>
                            <GeometryDrawing Geometry="F1 M 35,-124C 45.2172,-124 53.5,-115.717 53.5,-105.5C 53.5,-95.2827 45.2173,-86.9999 35,-86.9999C 24.7827,-86.9999 16.5,-95.2827 16.5,-105.5C 16.5,-115.717 24.7828,-124 35,-124 Z ">
                                <GeometryDrawing.Pen>
                                    <Pen Thickness="0.32" LineJoin="Round" Brush="#57008898"/>
                                </GeometryDrawing.Pen>
                                <GeometryDrawing.Brush>
                                    <RadialGradientBrush RadiusX="0.5" RadiusY="0.5" Center="0.71625,0.283751" GradientOrigin="0.71625,0.283751">
                                        <RadialGradientBrush.GradientStops>
                                            <GradientStop Color="#57FFFFFF" Offset="0.0104712"/>
                                            <GradientStop Color="#5712C5DB" Offset="0.65445"/>
                                            <GradientStop Color="#5714C5DB" Offset="0.654727"/>
                                            <GradientStop Color="#5717C6DB" Offset="0.905759"/>
                                            <GradientStop Color="#5711A5B7" Offset="1"/>
                                        </RadialGradientBrush.GradientStops>
                                        <RadialGradientBrush.RelativeTransform>
                                            <TransformGroup/>
                                        </RadialGradientBrush.RelativeTransform>
                                    </RadialGradientBrush>
                                </GeometryDrawing.Brush>
                            </GeometryDrawing>
                        </DrawingGroup.Children>
                    </DrawingGroup>
                </DrawingBrush.Drawing>
        </DrawingBrush>
    </sap:ActivityDesigner.Icon>