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 (#1)
Dude - I obviously already checked that. There's only one for the default window - not for buttons.
In response to Whitejedi (#2)
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.