UpdateBitmap throwing InvalidArgumentException

I set an image (16x16 bmp) to a BaseCommand when it is first created which works fine and shows correctly. However in the OnClick, I want to switch the image at runtime to a different image depending on when it was clicked. When I call base.UpdateBitmap I receive an InvalidArgumentException, however I tested this image by setting it as the startup image and it worked fine so it doesn't appear to be an issue with my image.

Has anyone else had a problem with UpdateBitmap or an idea what else could be happening?

Thanks