How can I add an image to a custom WiX dialog?

I have tried modifying the set of WixVariables in my Product.wxs like so:

<WixVariable Id="MainLogoBmp" Value="Resources/Images/weblabel.jpg" />
<WixVariable Id="WixUIBannerBmp" Value="Resources/Images/installer_banner.jpg" />

(The first part is what I tried, the one below it is an example of the stock variable that works)

...and then referencing the variable with !(wix.MainLogoBmp):

<Control Id="Bitmap"
         Type="Bitmap"
         X="0"
         Y="0"
         Width="258"
         Height="185"
         TabSkip="no"
         Text="!(wix.MainLogoBmp)" />

...but when I try to compile this I get the following error:

Error 17 ICE17: Bitmap: 'Resources/Images/weblabel.jpg' for Control: 'Bitmap' of Dialog: 'SimpleDlg' not found in Binary table

And yes the image is part of the project, set to "Content" like the other ones.

How can I add an image to a custom WiX dialog?

1 Answers