Google added to the help documentation for the logo schema/structured data that you can now use ImageObject type to specify an organization logo. This is in addition to just the URL type.
Google said this provides more flexibility for this schema.
Previously, the help documents only specified the URL type for logo schema.
Why should an ImageObject type be used instead of URL type? If you want to provide data about the image, you have to use ImageObject because you can't add properties to an URL value. Such data could be the height and width of the image, or its author, or its caption and so on.
More on this ImageObject can be found on Schema.org.
Google also added a guideline that reads "make sure the image looks how you intend it to look on a purely white background (for example, if the logo is mostly white or gray, it may not look how you want it to look when displayed on a white background)."
Forum discussion at Twitter.
