Putting Images Into the Media Section
On the left side of this page, you'll see a long list of options. If you click Media, you'll be taken to a page full of images.
If you want to add an image of your own (conforming to the guidelines, naturally), you can do so by clicking the Add Media button.
The website will then give you a choice of which section to put your image into. There's only one section: Fan Art, so click that.
This is a topic on uploading images so click on the Upload file link.
Now pick your images to upload. There's a max size of 5MB per file and you can only upload so much in total. As long as you stick to JPG files, you should be fine.
The image(s) will be displayed to you in a page that lets you edit the title, description, and key words for the file. (Key words are good for searching. The game's name, maybe the publisher's name, or even character names are good to put in. Whatever you think people might search for that isn't already in the title.)
Click Save and you'll be taken to the image's page.
Putting Images Into a Forum Post
The image's page has the key to this in the lower right, where you'll find all sorts of options for sharing your image. The one you want for posting here is the Copy Image BB Code. Just click the little icon next to it and the BB code will be copied into your Windows clipboard.
Now go to the forum post you're writing and simply paste it into the post. You can do a preview to see how it will look.
There's another option right below the 'Copy image BB code' to do similar but with a thumbnail. This is handy if you've got a bunch of images to include. Putting a lot of images (say, more than 6?) can be a strain on people's bandwidth. Not everyone has gigabit connections, you know! With the thumbnails, you can show small versions of the images and people can click on whatever images they want to see.
People who click on images get to see the image fill their browser with a black background behind the image. There are various icons in the upper right so they can see the full image (or just download it). If you know your BB code, it's possible to make it so clicking your image goes to the full image instead, but I don't think there's really a need when posting on these forums. You can also use width or height in the BB IMG tag if you want to mess with the image's size if you don't want to fill up the post but don't want to use the thumbnail. See the BB help for that (or, if you know HTML tags, you can figure it out with a little guesswork).