There are two different issues.
ISSUE 1: Repeated Use of Wrong Image
An image must be representative of the business. Using one for all entries is therefore wrong.
I do not know for sure, but I doubt you would be penalised. But (per issue 2 below) there is no reason to add an image at all.
In fact, adding incorrect values may confuse Google and others and so make it harder for the desired associations to be made.
Finally, you are making an association between your clients (the businesses) and some random image which:
they may not want/appreciate;
may even hurt their own seo efforts; and
will decrease the value of your site's citations.
ISSUE 2: Snippet Validation is NOT Schema Validation
Google's validator is for SNIPPET display - i.e. what schema is required for them to display the LocalBusiness SNIPPET on their search results page.
This should NOT be the objective your site. That should be the object of the submitting site. Your objective is citations - completely different.
Even though your markup doesn't pass "snippet validation" without an image property, it is still valid schema and is useful for your citations efforts because it will almost certainly still be used by Google and others to make associations between your page and the relevant Local Business - which is what you want.
So just don't bother adding any image property at all.
EDITED: To try explain more clearly.