1.
First thing to do is to load up your build and take some screenshots of it. Turn on image effects for a better look. You find this under the graphics settings. The images your take should automatically save onto your device.
2.
Now you find the best of the images you have taken and edit it the way your want. This is optional, but recommended. Once your done with this it's time to open up Simpleplanes.
3.
Once you got Simpleplanes opened you need to open the build menu (the place you normally build and release your builds). Now load up the build you want to release on the website. For this example i'm gonna use my Saab 340.

Then you open up blueprint veiw. You find this under the eye icon:

Then you press this and choose which side you want the see the build from.
Just select one one of them. This does not matter.
Then you can add a picture by clicking open image. This will open up the place where all the pictures on your device are stored. Then select your thumbnail pic.

Then the picture will appear in game like this:

Select the arrows and move it away from your build so nothing is covered.

Now you share your build and when you then want to take pictures of the build you zoom in on your thumbnail pic.

Make sure only to zoom in and out. If you change the perspective your picture will disappear.

And now you release the build and bam. You got yourself a nice looking thumbnail!
Let me know in the comments if there is something you don't understand.