You could change your xaml like this instead:

<Image>
    <Image.Source>
        <BitmapImage CacheOption="None" UriSource="your path" />
    </Image.Source>
</Image>

to be able to play with the CacheOption parameter, to specify how the Xaml Builder should load the image file, as the default value seems to be keeping a lock on it (waiting for the GC to do its work it seems).

Here is some related question here on SO: How do you make sure WPF releases large BitmapSource from Memory?