I think what Markus told is the one way to resolve your issue and try by adding one more property in it RenderOptions.EdgeMode="Aliased" for each image I mean :
<Image Source="/LoginPanel;component/Icons/icoLogin.ico"
RenderOptions.BitmapScalingMode="NearestNeighbor"
RenderOptions.EdgeMode="Aliased"/>
if you still not able to fix your problem then you can refer this http://blogs.msdn.com/b/dwayneneed/archive/2007/10/05/blurry-bitmaps.aspx to create a custom Bitmap class and apply on all Images which are creating trouble for you.
You can also see this Stack Overflow Question