Avoid errors when image size is empty

Hi,
i've found that there were a problem when a news has a non "standard" image (for example a bmp or a tiff).
The utils can't get his size, and tried to create a miniature passing width="" and height="", and generate a bad Pillow error: MemoryError.

The strange thing is that with images stored in blob, this doesn't happens.

The workaround is to return None if we can't get the sizes. This could be related (and close) the issue #31

P.S.: some tests don't pass for a buildout problem