There are 2 ways. Either by using the "Manage Attachments" function to upload the image from your computer (size in pixels and KB is important) or if the image is located on another website, you insert the URL of the image, not the webpage the image is on, in your post. The icon for that is the yellow rectangle thing in the same line as the BOLD,italic, and underlined icons. The following might help you more:
FAQ - Images and Attachments
Searching the Discussions about this site subforum for other threads asking your question will also provide you with additional explanations of how to attach images.