Schema.org does not require any properties, so having a
BlogPosting without an
image property is totally fine. (And, other than omitting the property, there is no way to denote that you don’t have an image.)
(I’d even guess that not providing an image would be the norm, as the
image property is not for just any image that is contained in the blog post, but for an image of, or an image that represents, the blog post itself.)
The fact that Google reports this error in their Structured Data Testing Tool does not mean that the code would be invalid or that you would have to fix anything. (Ideally Google would mark these as informational, or warnings, but not as errors.)
It just means that Google won’t provide an Article Rich Snippet for your blog post in their results, as the
image property is required for that, according to their documentation.
If you just want to check your Microdata and don’t care about Google-specific stuff (like their Rich Snippets), have a look at alternative tools: Online Microdata parser (Software Recommendations SE).