Some questions............
Post card as in marketing/reminder tool, I assume??
Regular postcard size?? Dimensions? Who are they going to/aka, the target audience for the post card. New customer, as well? e-Mail or electronic distribution?
Obviously don't know what you've done before, but will the "correction" photos even show up on a post card?? Printing will have to be VERY good in hard copy to show that. I'm sure the resolution is good enough in the online photo, but what are you going to have it printed on/what device will print it?
The cars are cool............depending on your "message", likely better choice.
(BTW/FYI - I see this all the time on the site. If this is a hard copy distribution - the quality has to be excellent, therefore, the piece has to be designed to allow that level of quality. So, the first thing I would ask about any marketing collateral like this - is, what is it's purpose? Does the expense justify the probable results? And, And, probably said in a better way - "what are your objectives?" Let that drive the piece design).
Of course, that's just my opinion.................I could be wrong
See ya. :wavey
PS - the idea of a promotion is a good one to consider................XX% off with the card; a free service with the card; bring a referral and get XX%/$ off; $'s off on the "next" detailing service in the spring, etc., etc.. Something of value for the customer - perhaps low cost/effort for you, but something appreciated by the customers.