Say, for example, a player creates a cool looking car that is customized with colors and attachments the player applied. I would like to be able to take a thumbnail picture of that car. That way, when showing the player their inventory of cars, a small thumbnail image shows what each car looks like.
So I would like to programmatically create the thumbnail image. Is that possible?
As far as I’m aware, no.
This is complicated solution, but you could raycast each pixel to determine the color, export via HttpService, and use some kind of bot to upload the image to Roblox.
I think I once visited a place where you could click a button, which would then create a new place using the current state of the game, and find that place’s thumbnail to use.
Quite old stuff though, https://devforum.roblox.com/t/using-place-thumbnails-and-saveplace/8172 - I haven’t had any luck finding something closer than this, unfortunately. 
It’s a better pratice, if you can, to show the rendered model of the car itself, or a simplified version of it, using “3D GUI” modules. If you want the actual image asset, you need to upload it from either a bot or using the CreatePlace API, if you’re using the API you may need to use model thumbnails and get the image used in the game icon.
This could easily get you banned if a malicious player were to upload something inappropriate that gets through the auto-moderation.
I’m pretty sure this used to be a thing with personal building servers, however I haven’t seen anything like it since they were removed.
I need something like this so if somebody would make a feature request for it that would rock.
A massive use case for me is showing images of players in-game as their custom characters, since they change them in my game. It’s weird to show their normal avatar when they won’t appear like that in-game.
Every year Roblox employees participate in hack-week, and this was one of the projects that was presented to everyone. Will it be made into a feature? There are awesome advantages to it but also some concerns already mentioned above. In addition, it takes time to develop and away from other features. It was presented February or March 2018 I believe.
What I’ve done(which is super unprofessional), is I’ve screenshotted where I want, crop to the correct resolution(avoiding the UI obviously), gone into paint.net, made sure my cursor was over a surface that is mostly all the same color, and just painted over the cursor to match the background.