Here is a short tutorial about gridcar-editing with Paint.Net.
I'm sure you can do this with every other good image-software as well.
1. Open your gridcar-image with Paint.Net.
2. I'm no expert in gridcars but it looks like as we have to switch the image horizontal and vertical here.
3. Then in Paint.Net there is a great feature called 'Zauberstab' (Engl.: 'magic wand', I'm sure in your language will also be a crazy word for it). With this feature you can mark a complete object by only one click. So here click the blue backround and press 'delete' on your keyboard. Paint.Net divides this image into two parts so you have to do this for the upper and the lower blue backround.
Explanation:
The more contrast you have in an image the better it is for this 'magic' feature. If contrast is too low or too high, try to change the tolerance-option above, use the rubber or make a manual mask (all options are greem-marked).
And don't worry: The chequered backround simply means that you have a 100%-transparency at that part of the image.
4.
The result should look like this:
Just save it and use GPI-browser to bring it in-game.
Kind regards
Klaus Feldmann