First you must delete the 1.9 before that
Make sure the have a backup files
Find this file on Android>>>Data

And then Put it in backup

Then delete the app also make you also have a backup 1.9 version

Then install The 1.8 version
if you don't have a 1.8 try find one in the Google

After that find the backup files and put it in Android>>>>Data

And done Now need some Mods
The most important Mod is the DesignerSuite

After Installing The mod open the game and activate the mod

Now let's go to the Main Topic

Now click the Ortho/Imagine

Then you realize Where's the image you supposed to use as a custom Thumbnail

Make the the Thumbnail You want and put in here and make use the image size is 16.9
Android >>>>Data>>>com.jundroo.SimplePlanes>>>Files

Then open this up again and find the Image after that it will load

But your Plane is on the way
So you must move the image to the Left/Right/Up Down/
By This

And then take a screenshot

And then post it

And the results is this

And done now you have a custom Thumbnail

Note: The down side of 1.8 is quite lagging and sometimes The screen went back and they don't have Cannon's and Funky Tree

Note:sorry for bad Engrish