It may not be obvious to everyone, as I see many people struggling with this, so I decided to make a mini-tutorial.

So you took an screenshot of your favorite mugen character and want to show it on the forum, or you made a cool image that you want to use as your avatar, but when you post it... it shows nothing! the forum is broken!
Don't worry, I'll guide you through this.

The first mistake I see some people doing is trying to show an image from their machine, rather than from a website or image host. This won't work, we can't see what is in your PC or phone. You must upload it to somewhere.

Back then we used sites like photobucket, tinypic and imageshack (RIP to all of them), they don't work anymore and that's why you'll see some missing images on older posts.
Now, the most used image host and the one we recommend is imgur:

https://imgur.com/

Create an account on imgur if you don't have one yet. You'll see a button to upload images in the top left corner, it says "+ new post".
From there you can upload any image from your PC or phone. You can even organize images in albums if you want.

Once you upload your image, you'll see this screen:


What some people do wrong is that they copy this page's link instead of the image link, which looks like this:
https://imgur.com/WDSrh7L

This won't work, as this is the WEB PAGE's link, not the image link. Instead, you need to right-click the image and hit "copy image link" or "copy image url" (it may be different depending on the browser). If you have done it correctly, your new link should look like this:
https://i.imgur.com/WDSrh7L.gif

Notice how it ends with ".gif" here. This is the correct link. Keep in mind that your link should end with .jpg, .png or .gif.
Another way to get the right link is to click on your profile in the top right corner, then go to "images", find the image you just uploaded and click it. You can now click to copy the Direct Link in the right side:


Ok, you have the right link copied! What now?
To post it on the forum, you can click the little "insert image" button, which will add "[img][/img]" to your post, or you can simply type it yourself, then add the link between the two tags, like this:
[img]https://i.imgur.com/WDSrh7L.gif[/img]

Result:


And it's done!
For your avatar on the forum is the same thing, except you won't need the [img][/img] tags. Just remember that your avatar image must be 150 x 200 pixels at maximum, or it won't show up!

Also worth mentioning... DO NOT USE IMAGES FROM DISCORD! They vanish after some time. Imgur is still the option as of now.