I'm still investigating the matter, but in addition to Seans suggestion, you could alternatively you inherit from the image control, hard-code the layout template path by overriding the property value to point to your template, then add that control to the toolbox with a different name.
This is a bit of an extreme workaround, since if this is a bug and gets resolved in the next release you'll have to go back and replace them all again to use the toolbox property...
but it's an option! I'll get back when I know more about why this isn't working the way you expect...