Any photo that you wish to display must already exist online. There are many free online storage websites that allow you to store your photos online, and then display them on the forum. Here is a short list of a few of them:

Picasaweb (run by Google)

Flickr

Smugmug

Photobucket

There are many more. Just search the web for “free photo hosting”. Also, most internet service providers give you free space on their servers to use for personal websites or file storage, which you could also use to host your photos.

Once your photos are online, just copy the URL of the image you wish to post, press the “Insert Image” icon  on the forum posting page, and paste the URL into the box. You can configure the look of the image by adding a description, a border or defining the alignment.

By default, your image will be resized to show it within the forum's margins. Clicking on the posted image shows a full-size pop-up.

Click on the Register button at the top of the page,

Then fill out your details and await the confirmation email. Then log-in with your username and password and you are good to go.

Navigate to the forum that you want to post in, and click the "Add a New Topic" link. You can type in plain text, or you can format your post using the WYSIWYG editor.