1. Chose a screenshot you want to make into a thumbnail.
   
2. Crop this photo using a perfect square aspect ratio so all four sides are the same length. If you are in windows 10, this can be done with the default photo viewer.
   
   • Open photo and select Edit > Crop and Rotate > Aspect Ratio > Square.
3. Now you must resize your photo to exactly 256x256 pixels. Again, if you are in windows this can be done with Windows Paint.
   
   • Open your image with Paint.
     
   • Select "Resize" from the top toolbar.
     
   • Click resize by "Pixels."
     
   • Change "Horizontal" and "Vertical" to 256. (Maintain Aspect Ratio should be selected. This is why it's important to crop your image into a square.)
     
   • Save your image.
4. Now you must convert the image to .png (portable network graphics). This can again be done with Windows Paint or you can search the web for .jpg. to .png converter. Open image in Windows Paint > Save As > Select .png from the "Save As Type" list > Save your file.
   
5. Rename your file "Thumbnail.png" (This is important for it to work).
   
6. Now we must replace the old thumbnail with our newly created one.
   
   • Open Steam > Go to your library > Right click on Software Inc. > Select Properties > Local Files > Browse Local Files
     
   • In the window that opens, open the "Blueprints" folder.
     
   • Locate the folder with your blueprint name and open it.
     
   • Copy and Paste your created Thumbnail.png into this folder. Overwrite the previous file here.
     
   • NOTE: Save a copy of the Thumbnail you made. Anytime you edit this blueprint the thumbnail will be overwritten and you will have to replace it again.
7. To test if this worked, you should see your thumbnail when you select the blueprint in game.
   
8. Now when you upload your blueprint to the workshop it will have the thumbnail you created! If you have already uploaded your blueprint to the workshop, you can try to go through the upload process again which will update the blueprint not erase it.