In order to change the thumbnail from the default provided, navigate to the install location of Solar Nations.
If unsure of where the game is installed, go to your library and right click on Solar Nations in the listing. Navigate to Properties -> Local Files and click "Browse."
Once you have found the install location, navigate to Windows\solarnations\Saved\Icons. There you will find multiple folders and an image named "thumbnail.png." Replace this image with the thumbnail of choosing before uploading a mod.

While we are here, these folders are where the icons in Solar Nations are loaded from, so if you want you can put new icons or flags here and they will show up in the game. You can even use these icons in your mod and they will be automatically packaged with the mod when it is uploaded, so no heavy lifting is required there.

Before we can upload anything to the workshop we must first decide what exactly we are uploading. There are two things which can be uploaded to the Solar Nations workshop: Scenarios and Define packs.
Scenarios should be self explanatory, they are the custom scenarios you can make in game using the Scenario Editor. To get to the Scenario Editor, simply open the pause menu when you are in a single player game.

Scenarios
To upload a Scenario, navigate to the section of the scenario editor which allows you to change the background, difficulty, and filename of the scenario.

Define Packs
Defines packs are mods created using the Defines Editor located in the Mods section of the title menu. In order to navigate to it, either select or create a local mod and go forward from there.
When it is time to save your mod, the selection system is similar to that of scenarios.

In either case, pick a filename which you imagine will be distinct for your mod, as multiple mods with the same name will overwrite each other. Then, once everything else is covered, click the "upload" button at the bottom of this menu, which will prompt you to either overwrite an existing workshop submission or create a new one. Most likely, you will create a new one, but before you do this, you should change the thumbnail to distinguish your mod from others on the workshop.

Once you have set the thumbnail to the desired image, you may now upload your mod. Wait until the workshop item list closes before closing the GUI or exiting the game, as it can take a little bit for the upload to register on steam. After the upload has finished, it can take many minutes for your changes to register on Steam, so be patient if it is not working.

Sometimes Steam can be finnicky and give an "access denied" error, which will prevent you from making any new mods for a couple of hours. In order to check if this is the case for you, navigate to a mod for Solar Nations you have already made, and attempt to edit its name or description. If you are unable to do so, you should wait at least 2 hours before trying again.

Once your changes go through, you should now have an unlisted Steam Workshop item with a thumbnail and title resembling that of the File Name of the content pack you uploaded. If you don't know how to find an unlisted item, navigate to your profile and then your content, your published items for all games should show up there. You can edit the description and title of your item as you please, but remember to change the item's visibility if you intend on distributing the content to others.

You can also upload more images of gameplay or screenshots of the mod using Steam's tools, but this is identical to all other workshop submissions so I won't go into detail about it.

This part of the guide deals with how to use items you have installed, made by yourself or others.

The most important thing to remember about Item usage in Solar Nations is that the remote items are identical to and thus will overwrite local items of the same name if enabled. Therefore, you should make backups of your SaveGames folder any time you mess around with mods, as some funny man might name their mod "manual_save_0" and thus overwrite your first save file (I will fix this eventually). In addition, other scenarios and mods can be overwritten by each other so make sure you are only enabling the mods you need.

If you intend on playing multiplayer, you must enable the same mods as the host otherwise the game will likely kick you out. Unfortunately, even if you have the same mods subscribed you must still enable them for the game to recognize that they exist.

Fortunately, this can be done easily by navigating to the title screen mods selector. Here you can see a list of each mod you are subscribed to, as well as whether or not they are enabled. Enable the items you wish to use and remember to always re-enable your mods whenever you relaunch the game, as it does not save which ones are enabled.

ProTip: Definitely do not name your mod CoreDefines