To post an image here on A2K, the image must first be "On the Web" - have a URL, or "web address". You can upload an image from your machine to any of a number of host sites which support what is called "Hot Linking", or the use of images hosted on that site as displayed images on another site; the image really isn't on A2K - the BB code causes the machine of anyone viewing the post to request the image from thje website on which the image actually is hosted. Not all hosting sites support hot linking, but many do, some free, some subscription, as you prefer. Once the image has been loaded to a host, its URL can be used, with the appropriate (and automated) BB code to cause it to appear in a post on A2K. OK - that's the short version.
The longer, much more detailed version is available at
Tutorial - How to Post an Image - that thread contains everything from the basic mechanics and suggestions for hosting sites and graphics editing software to tips and tricks that go way beyond the basics. Take it at your own pace, and ask if you have trouble or questions - share your learning experience with others, and before you know it, you'll be teaching others.