Djwasim,
As far as I know, this is not possible. You either need to make the size of the button to accommodate the size of the image, or you need to use an image editing utility to create a different sized version of your image.
The exception here would be if you can use a properly configured icon (.ico) for your button, in which case the appropriately sized image from the icon set will be deployed to the button. There is some software that can be used to produce icons from images in other formats. I use Axialis IconWorkshop and can recommend it.