Here is the Simviation Banner that you will see at the top of any Forum Page.

You can see that I have highlighted the Uploading Images Link.
When you click this link The Uploader opens in a new tab.

Now make sure you read the Guidelines:
Allowed Types: jpg, gif, png
Max size per file: 155kb. 1024 pixels wide
Max size for all files combined: 650kb.
Max size for Avatar images: 15kb/120 pixels wide
Max size for Signature images: 30kb/600 pixels wide
Now click the Browse Button. Depending on your Operating System the Pop-up may or may not look like this.

Now browse to the Picture you want to Upload & Select it.
When you have selected all the Pictures you want click Upload File(s).

Now Highlight the Code inside the [img] tags & paste the code into the message you want to put the picture in.