For most of the time I've been stealing in-game originals but they don't match well on some of my newer projects so I would now start trying to make my mods looking more professional and this looks like a good start.
well if you've been nicking original thumbnails, you should know its just a .png named the same as your config. So just take a screenshot in png, resize it to the same size as default thumbnails and tada
Still I kinda would like a black outline with a professional look I've seen it be done so I know it's possible.
There are plenty of free alternatives out there: For example gimp: 1. Open you picture with gimp 2. Put your car/selection/whatever on a new layer 3. Rightclick on that layer and choose 'alpha to selection' 4. Rightclikc on the new created selection adn choose 'Edit - Stroke Selection' 5. Choose your color in the toolbox and click on 'stroke'
Why editing? Just go to dm showroom or something similar and take a screenshot with 15 fov. Then open it with any picture editor, resize and crop it to 250x120 pixels. I do that with my mod and it looks nice.