AS Studio: resizing icons on buttons

try dragging a 32 bit 128x128 icon onto a smallish button.
AS Studio doesn’t resize it but rather crops it in the middle. You can resize the button of course, and there is a Size to Fit option. But Size to Fit resizes the button to the image and not the image to the button.
The only work around I found was to delete the 128 x 128 preview image from an .icns file leaving only the 48x48 one. But you still can’t resize the image–you’re stuck with the largest image in an .icns file–I think.
Am I missing something here?