Hello and welcome to my first Guide! We all know thumbnails are a huge pain to get uploaded correctly because the HOI4 Launcher will upload the Mod but will not tell you what was wrong with your thumbnail! This Guide is here to help with that! I will just tell you how I upload my thumbnails!

Step 1: Standardise naming. Always name your thumbnails "thumbnail.png" no matter what. Save it to the Main directory of your Mod where also the descriptor.mod file is located.

Step 2: Standardise size: Make sure your picture is not too big in size! If you have a thumbnail that is lets say 600x600 then it won't work and will never tell you why it didn't work. I found 499x499 maxiumum to be a safe bet.

Step 3: Reference the thumbnail in the descriptor file in your Mods Main Folder. Make sure you open and edit the decriptor file with Notepad++ and not just the regular Notepad! The reference should be:
picture="thumbnail.png"

Example given:
...
name="YOURMODNAME"
picture="thumbnail.png"
supported_version="1.9.*"
...

(Make sure you press "Save" in Notepad++ and not "Save As" as this will give you some annoying errors. Just open, edit and press save.)

Step 4: Also put the same reference into the descriptor file in the HoI4 Mod folder (yourmodname.mod)

Step 5: Now start the launcher, select your mod, write your description and press upload and now your mod should have a proper thumbnail!

Have fun and good luck with creating your own Mods!

PS: This is only my first Guide so if you have any wishes for future topics you'd like to see me cover feel free to tell me in the comments!