ImageSource and ImageBrush events can cause a memory leak
For example, the code below creates and discards a user control name "ImageControl":

    public partial class Page : UserControl
    {
        public Page()
        {
            InitializeComponent();
            new ImageControl();

        }
    }

... where the user control is defined as:

    public partial class ImageControl : UserControl
    {
        public ImageControl()
        {
            InitializeComponent();

            var imageSource = new BitmapImage(new Uri("http://contoso.com/"));
            imageSource.DownloadProgress += imageSource_DownloadProgress;

            var imageBrush = new ImageBrush();
            imageBrush.ImageFailed += imageBrush_ImageFailed;
        }
    }

As written above, the BitmapImage and ImageBrush leak.  The event listeners cause the whole ImageControl to leak.

These are "soft leaks", because they do go away when you reset the core (navigate or refresh in the browser).

Workaround: The workaround is to remove the event listeners, at which point the ImageControl can be collected, though not the ImageSource and ImageBrush. 

If the above Page is created in Xaml, then removing the event listeners also makes the ImageSource and ImageBrush collectable.  An example of the above page in Xaml:
    <UserControl x:Class="ImageSourceEvent.ImageControl"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Width="400" Height="300">
        <Grid x:Name="LayoutRoot" Background="White">
           
            <StackPanel>
                <Image Name="_image">
                    <Image.Source>
                        <BitmapImage x:Name="_bitmapImage2" UriSource="http://foo.com/"

DownloadProgress="imageSource_DownloadProgress" />
                    </Image.Source>
                </Image>
               
                <Rectangle Name="_rectangle">
                    <Rectangle.Fill>
                        <ImageBrush x:Name="_imageBrush2" ImageSource="http://contoso.com/" />
                    </Rectangle.Fill>
                </Rectangle>
               
            </StackPanel>

        </Grid>