Sounds like an excellent defense to me, especially if you have a picture showing that the only markings are on the pavement.
What you need to look at is the specific city ordinance that you are charged with. The statute number should be on the ticket.
I found where an elevated sign is required
e) Special parking locations and privileges for persons with disabilities that limit or impair the ability to walk, also known as handicapped parking spaces or disability parking spaces, shall be provided and designated by all political subdivisions and by the State and all agencies and instrumentalities thereof at all offices and facilities where parking is provided, whether owned, rented, or leased, and at all publicly owned parking garages. The locations shall be designated through the posting of an elevated sign, whether permanently affixed or movable, imprinted with the international symbol of access and shall be reasonably close to exits, entrances, elevators, and ramps. All elevated signs posted in accordance with this division and Ohio R.C. 3781.111(C) shall be mounted on a fixed or movable post, and the distance from the ground to the top edge of the sign shall measure five feet. If a new sign or a replacement sign designating a special parking location is posted on or after October 14, 1999, there also shall be affixed upon the surface of that sign or affixed next to the designating sign a notice that states the fine applicable for the offense of parking a motor vehicle in the special designated parking location if the motor vehicle is not legally entitled to be parked in that location.
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