Vista - Gif Resource, Drawing namespace not found

I'm trying to place a .gif on a button. Like this:

<Button Width="80" Height="25" Click="GeneratePuzzle" >
<StackPanel Orientation="Horizontal">
<TextBlock Margin="0, 0, 10, 0">Generate</TextBlock>
<Image Source="{StaticResource GoButton}" Width="14" Height="14"/>
</StackPanel>
</Button>

And I've added the image through the Resource.resx, 'Add Existing
File', and named it GoButton.

However, when I try to build I get the error in the
Resource.Designer.cs file:

Error 2 The type or namespace name 'Drawing' does not exist in the
namespace 'System' (are you missing an assembly reference?)

The offending code is:

internal static System.Drawing.Bitmap go {
get {
object obj = ResourceManager.GetObject("go",
resourceCulture);
return ((System.Drawing.Bitmap)(obj));
}

My question is, what do I need to do so the generated code can find the
namespace, or is there a better way to include a gif file like this?