Continuing on:

When you have clicked the 'Set View' for the image that you opened it up on, it should have appeared like this:

You then want to use the offset tool to move the image upwards so you can get a clear screenshot when you upload your build, you can also scale the image to as large or small as you want:

The final step is to click on the 'Share Aircraft' button in the SP designer main menu so you can screenshot the thumbnail image:

Then you screenshot the image only, making sure you can't see anything else. The black bar does disappear when you take the screenshot, so don't worry.

And Control Alt Bingo, you're done. Just upload your build when you're ready and it will have the custom thumbnail, just keep in mind that the thumbnail image is the one that is at the top of the upload screenshot list.

Hope this helps for anyone who doesn't know how to do this :3