BlogPosting Publisher Logo 'logo.itemtype has an invalid value' on Google Structured Data Testing Tool

Turns out, because BlogPosting is one of the types supported by Google as a possible Rich Snippet, they apply more validation:

Google Search Documentation Guidelines for Articles

This requires an Article's Publisher's logo to be of type ImageObject and have a width and height in pixels. BlogPosting is a subtype of Article.

This updated snippet validates through the Google Structured Data Testing Tool:

<div id='web-page-example' itemprop="mainEntityOfPage" itemscope itemtype="https://schema.org/WebPage" itemref="headline-example">
    <div>
        <div itemprop="publisher" itemscope id="organization-example" itemtype="https://schema.org/Organization">
            <a itemprop="url" href="https://example.com">
                <span itemprop="logo" itemscope itemtype="https://schema.org/ImageObject">
                   <img itemprop="url" src="https://example.com/images/logo.png" alt="LOGO">
                   <meta itemprop="width" content="600">
                   <meta itemprop="height" content="60">
                </span>   
                <span itemprop="name">EXAMPLE</span>
                <span itemprop="description">This is an EXAMPLE</span>
            </a>
        </div>
    </div>  
    <div
        id="blog-posting-example"
        itemprop="mainEntity"
        itemscope
        itemtype="https://schema.org/BlogPosting"
        itemref="organization-example web-page-example"
    >
        <span itemprop="author" itemscope itemtype="https://schema.org/Person">
            <span itemprop="name">Example Author</span>
        </span>
        <time itemprop="datePublished" datetime="2016-05-09T11:40:04+02:00">9th May 2016</time>
        <time itemprop="dateModified" datetime="2016-05-09T11:40:04+02:00">9th May 2016</time>
        <h1 id="headline-example" itemprop="name headline">Example Headline</h1>
        <span itemprop="image" itemscope itemtype="https://schema.org/ImageObject">
            <img itemprop="url" src="https://example.com/images/blog.png" alt="LOGO">
            <meta itemprop="width" content="800">
            <meta itemprop="height" content="400">
        </span>
    </div>
</div>  

A brilliant and helpful answer by @Arth above.

To complement the answer above (not compete with it), here is the same Structured Data using the same schema.org vocabulary, but this time in JSON-LD:

    "publisher": {
        "@type": "Organization",
        "name": "myOrganization",
        "logo": {
            "@type": "ImageObject",
            "name": "myOrganizationLogo",
            "width": "60",
            "height": "600",
            "url": "http://my-organization.org/my-logo.png"
        }
    }

N.B. According to https://developers.google.com/search/docs/data-types/articles

1. The logo should be a rectangle, not a square.

2. The logo should fit in a 60x600px rectangle., and either be exactly 60px high (preferred), or exactly 600px wide. (For example, 450x45px would not be acceptable, even though it fits in the 600x60px rectangle.)