I don't "like" Gunma's post, but I do agree. HOWEVER... I'm wondering if perhaps there might be templates, with areas that can be colored, and decals, names and racing number/league plaques applied. That at least would be something, enabling us to differentiate our cars from the field.
I must admit though that I'm holding out hope for at least something as basic as the Ferrari Challenge Livery Editor, which was something in-between Forza 1 and 2 in capability. Rather basic, but you could create some nice liveries with it. And it gave you the capability to put your car number on the windshield.
For GT7, go all out like Forza and certain other games do, down to having the capacity to make custom decals and league number plaques. I'd love it if PD could figure out an elegant way to go "sideless," as a system Eutechnyx was working on for the follow up game to Supercar Challenge. This way, you could slide decals and vinyl shapes all over the car's surface and not have to fuss with boundaries cutting them off. That may be a bit too difficult to pull off though.
For those wanting to import images, one solution would be for some PSN admins to approve them. Send them in an attachment, a mod looks it over, and sometime later, it shows up as an additional graphic element in your GT7 decal list. And it would be great if you could share it.