The goal is to create our own custom workshop thumbnails and frames for our creations in Stormworks.

When uploading our creations, Steam uses a default craft image thumbnail. The picture is not very flattering so if we hope to get noticed on the workshop it helps to have a nicer looking thumbnail for our craft.

The thumbnail includes both a background image and then a frame is applied on top of it to add the yellow tab and black band with "VEHICLE" printed on it. First we will change the image, and once we understand how to do that part, then we can talk about options for applying custom frames.

We start with a screenshot of the craft from the game. Rather than just clicking the screenshot button, I usually press the ESC key and go into the camera menu so I can get a nice shot of the craft and hide the game UI elements. This is the beginner or starter boat, so the Starter Base Island makes for a nice background.

Next we load our screenshot from the game into our favorite image processing program.

I've used GIMP for years. It's probably the best free image processing software available. If you don't already have it you can check it out for yourself and download it here: https://www.gimp.org/

In the processing there are two key requirements for our thumbnail image:
1) Thumbnail size has to be: 512 x 512
2) Thumbnail image has to be saved in: PNG file format

So after a little editing in GIMP we have our new 512 x 512 PNG thumbnail image:

We have managed to find the save location of our craft files here:
C:\Users\[username]\AppData\Roaming\Stormworks\data\vehicles

If you look in your vehicle directory, you will see the XML file for your vehicle and PNG thumbnail image. The logical thought is to just replace this PNG file with our new thumbnail image and presto!

I probably don't have to tell you... THAT DOES NOT WORK!

We are going to ignore the above Stormworks saved vehicles directory and instead look here:
C:\Users\[username]\AppData\Roaming\Stormworks\thumbnails

Note: If you can't locate the file using the Windows File Explorer, make sure you haven't allowed Windows to hide the directory. If I can help it; I don't let Windows hide anything! ...files, folders, file extensions, nothing!

When you open the folder, you will see it only has one "Default_Thumbnail.png" file. Go ahead and add your custom thumbnail image to this folder.

In the craft editor select the Steam Workshop Icon at the top.

Whether you are uploading a new craft or just updating an existing one, you will see the following workshop upload screen. ...and I circled something important.

Clicking on the defaut thumbnail image above will take us to the following Custom Thumbnail Selection screen:

In the image above, I drew a yellow rectangle around the reminder the devs left us for the image size and directory location. We select the custom image here that we want for our craft and...
...we see our custom thumbnail will now be used!

Final step is to just confirm:

Our coolio custom thumbnail is posted to the workshop!

Note: You do not have to delete your existing craft from the workshop and repost it to use a custom thumbnail. You can just update your craft and change the default thumbnail. There's no limit to the number of custom thumbnails you can store in that folder.

You may have noticed some thumbnails in the workshop will even go a step further and add their own custom frame. The frame files are located in the Steam installation folder for the game and can be found here: G:\Steam\steamapps\common\Stormworks\rom\graphics


The files we are interested in are the six "template_*.png" files selected above. The png file format includes the ability to save an alpha channel which allows transparency. Once again, we can edit these files with our favorite graphics program, but there are a few important points to be aware of when modifying these.

1) Template size has to be: 512 x 512
2) Template file type has to be: PNG
3) Your graphics program has to be capable of editing/saving the transparent alpha channel
4) Create a "template_default" directory in the graphics folder to save the old frames
5) Create a "template_custom" directory in the graphics folder to store custom frames
6) Now you can swap custom frames from/to the root graphics folder

There is no "Custom Frame Selection Screen" when uploading items. So if you want to use special frames for your craft, you have two options:

1) You could just move the templates from the graphics root directory into your "template_default" folder and just not include any files in the graphics folder. This way you can manage and apply your own frames in your graphics program. To me, this seems most logical.

Or...

2) You could modify and replace the existing frames. In the event that we are not able to delete them and the program just replaces the files if not detected, we could also just save an empty alpha png file for each of the templates and that would also effectively cancel them out.

If you do want to edit them, you can use whatever graphics program you prefer, but since all of these templates follow the same pattern, it is helpful if you can load and save them all as an individual project. Once again, GIMP is well-suited to this task!

It's easy to work with each template as a layer in GIMP. The selection wizards also let you easily save the non-alpha parts of the template image as paths so you can easily edit the existing frame with exactly the same dimensions and tab positions. Additionally, you can create folder groups for Default Templates and Custom Templates and keep everything organized.

Granted, you don't have to use the same areas for the black "VEHICLE" banner or the yellow tab, but if you are trying to match the format, the next step is to identify the font or at least find one with similar attributes.
Not sure what the exact font is that they use here. It's a sans-serif heavy block style with pointy tops and bottoms for the letters. The shapes of the letters "C" and "G" as well as the sides of the "M" being angled are most notable. The "Lemon Milk" font is a pretty decent match. You can check it here and download for free:
https://www.dafont.com/lemon-milk.font?text=ENVIRONMENT
Notice the "M" in the Lemon Milk font doesn't have the angled sides. ...but maybe we just flip the "W" upside-down?

I think for my custom thumbnails I might come up with my own system for frames. Maybe have one for "SUBASSEMBLY" or "CONSOLE" and I'm not a huge fan of the colored tab at top left, so I might just strike that completely.

Now you know all the tricks, so you can go create your own distinctive thumbnails and frames. Have fun!