Thank you very much.
Delete https://image.tmdb.org/t/p/original/u29ZjNsRrK9axnbI8Un4PdVvWnb.jpg

There is an App for Windows, better than MS Paint, similar to Photoshop.
It's free. That's what I use.
https://www.getpaint.net/

TIP: If you want to force the line break, click Space 2 times and then click Enter. Just Space > Space > Enter.

No worries.

In what way is it better?

(i.e. what's wrong with using MS Paint?)

Yes is better. More tools, more brushes.
For example in this case.
There is a tool that uses as an Ink any area of the image.
So you do not fill with a solid color.
And the brush size is variable. Then you can retouch the area seams.

Woud you be so kind as to point me to one of the tutorials on their forum. I don't even know what to look for.

I practiced Try and Err.
Maybe you should start by reading these pages: https://www.getpaint.net/doc/latest/

More attention to the tools.
https://www.getpaint.net/doc/latest/ToolsWindow.html
The two tools I use most are Rectangle Select and Clone Stamp.

What's the tool that "...uses as an Ink any area of the image. So you do not fill with a solid color."?

I want to be able to expand backgrounds on posters/backdrops that don't have solid color backgrounds. Example of image with background that is not a solid color: https://i.imgur.com/GUZmz9I.jpg

Clone Stamp

It is a stamp, which can vary in size and intensity (in menu bar).
For this example yours, I think it solves.
But, you need to experiment and learn using.
Take a test with this example.

Awesome, I'm now using that program!

Do you know if there's anything I can do on that program to prevent some image's colors/tone looking "dull" or "washed out" when I upload them after editing and saving them using Paint.net?
(They look fine when I view them after saving, but once they are uploaded to TMdb sometimes something happens where the images's color/tone changes, and the only way I've been fixing them is asking a mod to reupload them by giving them the source of my original image)

Edit: Btw, this problem is not specific to Paint.net, the same issue happens when I use MS Paint.

@tmdb24407783 said:

Awesome, I'm now using that program!

Do you know if there's anything I can do on that program to prevent some image's colors/tone looking "dull" or "washed out" when I upload them after editing and saving them using Paint.net?
(They look fine when I view them after saving, but once they are uploaded to TMdb sometimes something happens where the images's color/tone changes, and the only way I've been fixing them is asking a mod to reupload them by giving them the source of my original image)

Very strange. The same file.
You upload to TMDb and it gets bad.
A moderator uploads and stays ok.
How does the moderator get this file to upload? Imgur?
And when he's in Imgur can we see that he's okay?
It would be good to see some example.
Same file in:

• Imgur Link
• The upload link you made to TMDb
• Moderator upload link on TMDb

Here is one example (in this example I did not use MS Paint, I used Paint.net):

This is where I got the original image (3593x2433) from, and is also the link I provided for the moderator.

This is my upload.

This is the mod's upload.

~ ~ ~

Another example (in this example I used MS Paint):

This is the original image and also the link I provided for the mod.

This is my upload

This is the mod's upload.

I also have many more image uploads that have had their color/tone changed when I upload them but I haven't yet asked for a mod to reupload them. I want to first try and find out why this is happening. And if there is anything I can do to prevent it.

I downloaded the 3 images. The same defect appeared.
The flickr image has run out of color, like the image you uploaded. Lost chroma.
You can try to compensate in Paint.NET by using the tools:
Settings > Color / Saturation
Settings > Levels ...

How?

Did you try the one from TLAreleasing?

Sorry, the translation of the path was wrong.
Menu > Adjustments > Levels https://www.getpaint.net/doc/latest/Levels.html
Menu > Adjustments > Curves https://www.getpaint.net/doc/latest/Curves.html
Menu > Adjustments > Hue/Saturation https://www.getpaint.net/doc/latest/AdjustmentsMenu.html

What is TLAreleasing?

@tmdb24407783 said:

Another example (in this example I used MS Paint):

This is the original image and also the link I provided for the mod.

This is my upload

This is the mod's upload.

Same result. Just like what happens to you.