Title pretty much sums it up - Is there a way to make the image on a button stretch to fit the button, so if it is expanded, the image expands with it?
There is an option for that in the interface editor.
In response to Neimo
Dude - I obviously already checked that. There's only one for the default window - not for buttons.
In response to Whitejedi
Because you can't =D.

Unless it's just hidden on the interface editor. In that case you may be able to do it with some code.
I'm pretty sure you can't, I added a feature request for this months and months ago but there was never any response to it.