Scale images to prevent overflow/cropping

[BobVul]

From #2:

You might want to think about scaling the images. As you can see in my attachment, the standard notification display scales the image to fit the quadratic frame. This is only an issue if the source application uses non-standard aspect ratios, so not strictly necessary if you assume to work with applications keeping to standard growl formats.

I don't actually know if Windows will crop or scale down too-large images, so this needs some testing.

Could probably pad extra space with transparent background to preserve aspect ratio.