Hi Amigo, do you still need help with this?

Yes! Yes! Please

so which preview image do you want, one that shows up in game in the list or one that shows up in the workshop

If you want the preview for the workshop uploader program it must be 512x512 as you likely know. To do this, i take a screenshot larger than i need, then use the basic microsoft paint program and crop it to 512x512 then save it as preview.jpg on my desktop. I then select the jpg from within the workshop uploader and it works.
By the sounds of it you are trying to resize the image, this wont work as the image for the preview must be square but the screenshot you take is rectangle thus the sides will never be even when you resize. Cropping is the best way i found.
Another alternative is to make the longest edge 512. You can then add a blank white or black fill to the vertical to make it 512 also. Then you can use your text tool to put something there such as the map name or your name or logo. Just an idea.