Relative theme images not working in a user control

I've implemented theming on my site, using a combination of relative paths and SkidIDs. Everything is working great, except for one situation:
I have a user control that needs to refer to an image using relative paths, as the image url will need to change at bind time (it's an icon that's used to reflect a status).
<asp:Image runat="server" ID="imgIcon" ImageUrl='<%# GetImageFilename((byte)Eval("ForumThreadTypeId")) %>' ImageAlign="Baseline" />The GetImageFilname method simply returns "images/forum_thread.gif" at the moment (for testing purposes). If I try this in an aspx page, everything works as expected. I.e. the "images/forum_thread.gif" image is considered to be relative to the current theme in the app_themes folder. However, when I move this to a user control, the image path is now consider ...