Choose the machine you want to use (or build one specificaly for this) and proceed to the next step...

Do NOT upload your machine yet!!

Now, you probably noticed every time you open one of your machines it has a picture of what it looks like that is the thumbnail, the thing we are going to grab...

Close or minimize Besiege and open up documents (assuming you don't have a google PC) then choose this path: C:/Program Files (x86) (If you have a 32 bit PC, it will be in Program Files)/Steam/Steamapps/Common/Besiege/Besiege_Data/SavedMachines....

One you have got to saved machines open up the folder called Thumbnails, find the thumbnail that says your machine name and displays the picture of your machine. Then proceed to the next step

Copy the selected thumbnail and then paste it into your pictures folder for easier use, keep the name the same so it doesn't corupt anything in the future steps!...

Now open up your photo editing software, if you don't have any software here are a few suggestions for which ones to get: Paint.net (free), Sumopaint (free), Inkscape (free) the one I use is from the windows store called: PicsArt (free also)...

Import the thumbnail from the pictures (or if you remember the path use that but remember to replace the previous thumbnail with that one) and edit it to your hearts content!

Now that you have edited your thumbnail it's time to replace, grab the edited thumbnail and hit the copy button...

Now go to the path that takes you to the besiege savedmachines/thumbnails in step two. Once you are there paste in the thumbnail (make sure that the name is exactly the same with the old thumbnail), now if you have done it correctly it should come up saying would you like to replace the file(s), hit yes and wait for that to be done...

Lastly open Besiege for the last time and open the machine you chosen, it should have a new custom thumbnail! If so go on either save or open and hit the steam Icon...

Wait for steam to upload your machine, it will finish once the in-game browser opens up to your machine workshop page, now you have a custom thumbnail for that machine and have succesfully uploaded it too!

If you got this far thanks a lot for reading it! I hoped it help, please I know this is a little bit asking but could you please have a look at my newest Besiege machine, it would mean the world to me if you did that! Here is the Link just click this :D